Chapter 5

Next Generation Sunshine State Standards

Florida Sunshine State Standards Chapter 5

LA.910.2.2.3. The student will organize information to show understanding or relationships among facts, ideas, and events (e.g., representing key points within text through charting, mapping, paraphrasing, summarizing, comparing, contrasting, or outlining).

LA.910.4.2.2. The student will record information and ideas from primary and/or secondary sources accurately and coherently, noting the validity and reliability of these sources and attribut-ing sources of information.

MA.912.S.3.2. Collect, organize, and analyze data sets, determine the best format for the data and present visual summaries from the following:

line graphs circle graphs cumulative frequency (ogive) graphs

SC.912.N.1.1. Define a problem based on a specific body of knowledge, for example: biology, chemistry, physics, and earth/space science, and do the following:

1. pose questions about the natural world, 3. examine books and other sources of information to see what is already known, 4. review what is known in light of empirical evidence, 7. pose answers, explanations, or descriptions of events, 8. generate explanations that explicate or describe natural phenomena (inferences), 9. use appropriate evidence and reasoning to justify these explanations to others

SC.912.N.1.4. Identify sources of information and assess their reliability according to the strict standards of scientific investigation.

SC.912.N.2.2. Identify which questions can be answered through science and which questions are outside the boundaries of scientific investigation, such as questions addressed by other ways of knowing, such as art, philosophy, and religion.

SC.912.N.4.2. Weigh the merits of alternative strategies for solving a specific societal problem by comparing a number of different costs and benefits, such as human, economic, and environmental.

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Florida Sunshine State Standards Chapter 5

Overall Instructional QualityThe major tool introduces and builds science concepts as a coherent whole. It provides opportunities to students to explore why a scientific idea is important and in which contexts that a science idea can be useful. In other words, the major tool helps students learn the science concepts in depth. Additionally, students are given opportunities to connect conceptual knowledge with procedural knowledge and factual knowledge. Overall, there is an appropriate balance of skill development and conceptual understanding.

Tasks are engaging and interesting enough that students want to pursue them. Real world problems are realistic and relevant to students’ lives.

Problem solving is encouraged by the tasks presented to students. Tasks require students to make decisions, determine strategies, and justify solutions.

Students are given opportunities to create and use representations to organize, record, and communicate their thinking.

Tasks promote use of multiple representations and translations among them. Students use a variety of tools to understand a single concept.

The science connects to other disciplines such as reading, art, mathematics, and history. Tasks represent scientific ideas as interconnected and building upon each other.

Benchmarks from the Nature of Science standard are both represented explicitly and integrated throughout the materials.

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5Running Water and Groundwater

C H A P T E R

Rivers are a basic link inthe hydrologic cycle andrunning water is the mostimportant erosional agentshaping Earth’s land sur-face. This is the Cheaka-mus River in BritishColumbia. (Photo byRandy Lincks/CORBIS)

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116

All the rivers run into the sea; yet the sea is not full; unto the place from whence therivers come, thither they return again. (Ecclesiastes 1:7)

As the perceptive writer of Ecclesiastes indicated, water is continually on themove, from the ocean to the land and back again in an endless cycle (Figure 5.1). Thischapter deals with that part of the hydrologic cycle that returns water to the sea. Somewater travels quickly via a rushing stream, and some moves more slowly below the surface.When viewed as part of the Earth system, streams and groundwater represent basic linksin the constant cycling of the planet’s water. In Chapter 5, we examine the factors that influ-ence the distribution and movement of water, as well as look at how water sculptures thelandscape. To a great extent, the Grand Canyon, Niagara Falls, Old Faithful, and MammothCave all owe their existence to the action of water on its way to the sea.

FIGURE 5.1 Early morning on a lake near the village of Val-Morin in southern Quebec, Canada.Although the quantity of water in lakes is great, it nevertheless represents only a tiny fraction ofEarth’s total water supply. Check the graph in Figure 5.2 on the next page. (Photo by PatrickFrilet/Hemis/CORBIS)

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Earth as a System: The Hydrologic Cycle 117

Earth as a System: The Hydrologic Cycle

Sculpturing Earth’s Surface� Running Water

Water is just about everywhere on Earth—in the oceans, gla-ciers, rivers, lakes, the air, soil, and in living tissue. All of thesereservoirs constitute Earth’s hydrosphere. In all, the watercontent of the hydrosphere comprises about 1.36 billion cubickilometers (326 million cubic miles). The vast bulk of it, about97.2 percent, is stored in the global oceans (Figure 5.2). Icesheets and glaciers account for another 2.15 percent, leavingonly 0.65 percent to be divided among lakes, streams, ground-water, and the atmosphere (Figure 5.2). Although the per-centages of Earth’s total water found in each of the lattersources is but a small fraction of the total inventory, the ab-solute quantities are great.

The water found in each of the reservoirs depicted in Figure5.2 does not remain in these places indefinitely. Water canreadily change from one state of matter (solid, liquid, or gas)to another at the temperatures and pressures occurring atEarth’s surface. Therefore, water is constantly moving amongthe oceans, the atmosphere, the solid Earth, and the biosphere.This unending circulation of Earth’s water supply is calledthe hydrologic cycle. The cycle shows us many critical inter-relationships among different parts of the Earth system.

The hydrologic cycle is a gigantic worldwide system pow-ered by energy from the Sun in which the atmosphere pro-vides the vital link between the oceans and continents (Figure5.3). Water evaporates into theatmosphere from the oceanand to a much lesser extentfrom the continents. Windstransport this moisture-ladenair, often great distances, untilconditions cause the moistureto condense into clouds and toprecipitate and fall. The pre-cipitation that falls into theocean has completed its cycleand is ready to begin another.The water that falls on the con-tinents, however, must makeits way back to the ocean.

What happens to precipita-tion once it has fallen on land?A portion of the water soaksinto the ground (called infil-tration), slowly moving down-ward, then laterally, finallyseeping into lakes, streams, ordirectly into the ocean. Whenthe rate of rainfall exceedsEarth’s ability to absorb it, thesurplus water flows over the

surface into lakes and streams, a process called runoff.Much of the water that infiltrates or runs off eventually re-turns to the atmosphere because of evaporation from thesoil, lakes, and streams. Also, some of the water that infil-trates the ground surface is absorbed by plants, which thenrelease it into the atmosphere. This process is calledtranspiration Becausewe cannot clearly distinguish between the amount of water

to breathe2.spiro =across,1trans =

2.8%Oceans97.2%

Hydrosphere

Groundwater0.62%

Glaciers2.15%

Nonocean Component(% of total hydrosphere)

Freshwater lakes0.009%

Saline lakes andinland seas

0.008%Soil moisture

0.005%Atmosphere

0.001%Stream channels

0.0001%

FIGURE 5.2 Distribution of Earth’s water. When we consider only thenonocean component, ice sheets and glaciers represent nearly 85percent of Earth’s fresh water. Groundwater accounts for just over 14percent. When only liquid fresh water is considered, the significance ofgroundwater is obvious.

Evaporation320,000 km3

Precipitation284,000 km3

Precipitation96,000 km3

Evaporation and transpiration60,000 km3

Runs off36,000 km3

Oceans

Infiltration

FIGURE 5.3 Earth’s water balance. About 320,000 cubic kilometers of water evaporate each yearfrom the oceans, while evaporation from the land (including lakes and streams) contributes 60,000cubic kilometers of water. Of this total of 380,000 cubic kilometers of water, about 284,000 cubickilometers fall back to the ocean, and the remaining 96,000 cubic kilometers fall on Earth’s landsurface. Since 60,000 cubic kilometers of water evaporate from the land, 36,000 cubic kilometers ofwater remain to erode the land during the journey back to the ocean.

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118 C H A P T E R 5 Running Water and Groundwater

Running WaterSculpturing Earth’s Surface� Running Water

Running water is of great importance to people. We dependon rivers for energy, transportation, and irrigation. Their fer-tile floodplains have been favored sites for agriculture andindustry since the dawn of civilization. Furthermore, as thedominant agent of erosion, running water has shaped muchof our physical environment.

Although we have always depended on running water,its source eluded us for centuries. Not until the 1500s didwe realize that streams were supplied by surface runoff andunderground water, which ultimately had their sources asrain and snow.

Runoff initially flows in broad, thin sheets across theground, appropriately termed sheet flow. The amount ofwater that runs off in this manner rather than sinking into theground depends on the infiltration capacity of the soil. In-filtration capacity is controlled by many factors, including (1) the intensity and duration of the rainfall, (2) the priorwetted condition of the soil, (3) the soil texture, (4) the slopeof the land, and (5) the nature of the vegetative cover. Whenthe soil becomes saturated, sheet flow begins as a layer onlya few millimeters thick. After flowing as a thin, unconfinedsheet for only a short distance, threads of current typicallydevelop and tiny channels called rills begin to form and carrythe water to a stream. At first the streams are small, but asone intersects another, larger and larger ones form. Eventu-ally, rivers develop that carry water from a broad region tothe ocean.

Drainage BasinsThe land area that contributes water to a river system is calleda drainage basin (Figure 5.4). The drainage basin of onestream is separated from the drainage basin of another by animaginary line called a divide (Figure 5.4). Divides range inscale from a ridge separating two small gullies on a hillsideto a continental divide, which splits whole continents into enor-mous drainage basins. The Mississippi River has the largestdrainage basin in North America. Extending between theRocky Mountains in the west and the Appalachian Moun-tains in the east, the Mississippi River and its tributaries collect water from more than 3.2 million square kilometers (1.2 million square miles) of the continent.

River SystemsRivers and streams can be simply defined as water flowing ina channel. They have three important roles in the formationof a landscape: They erode the channels in which they flow,they transport sediments provided by weathering and slopeprocesses, and they produce a wide variety of erosional anddepositional landforms. In fact, in most areas, including manyarid regions, river systems have shaped the varied landscapethat we humans inhabit.

that is evaporated and the amount that is transpired byplants, the term evapotranspiration is often used for thecombined effect.

When precipitation falls in very cold areas—at high eleva-tions or high latitudes—the water may not immediately soakin, run off, or evaporate. Instead, it may become part of a snow-field or a glacier. In this way, glaciers store large quantities ofwater on land. If present-day glaciers were to melt and releaseall their water, sea level would rise by several dozen meters.This would submerge many heavily populated coastal areas.As you will see in Chapter 6, over the past 2 million years,huge ice sheets have formed and melted on several occasions,each time changing the balance of the hydrologic cycle.

Figure 5.3 also shows Earth’s overall water balance, or thevolume of water that passes through each part of the cycleannually. The amount of water vapor in the air at any onetime is just a tiny fraction of Earth’s total water supply. But theabsolute quantities that are cycled through the atmosphereover a one-year period are immense—some 380,000 cubickilometers—enough to cover Earth’s entire surface to a depthof about 1 meter (39 inches). Estimates show that over NorthAmerica almost six times more water is carried by movingcurrents of air than is transported by all the continents’ rivers.

It is important to know that the hydrologic cycle is balanced.Because the total amount of water vapor in the atmosphereremains about the same, the average annual precipitationover Earth must be equal to the quantity of water evaporated.However, for all of the continents taken together, precipitationexceeds evaporation. Conversely, over the oceans, evapora-tion exceeds precipitation. Because the level of the worldocean is not dropping, the system must be in balance. InFigure 5.3, the 36,000 cubic kilometers of water that annuallyruns off from the land to the ocean causes enormous erosion.In fact, this immense volume of moving water is the singlemost important agent sculpting Earth’s land surface.

To summarize, the hydrologic cycle is the continuousmovement of water from the oceans to the atmosphere, fromthe atmosphere to the land, and from the land back to the sea.The land-back-to-the-sea step is the primary action that wearsdown Earth’s land surface. In this chapter, we first observe thework of water running over the surface, including floods,erosion, and the formation of valleys. Then we look under-ground at the slow labors of groundwater as it forms springsand caverns and provides drinking water on its long migra-tion to the sea.

Students Sometimes Ask . . .Is the amount of water vapor that plants emit into the atmo-sphere through transpiration a significant amount?

Use this example to judge for yourself. Each year a field of cropsmay transpire the equivalent of a water layer 60 centimeters (2 feet) deep over the entire field. The same area of trees maypump twice this amount into the atmosphere.

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Streamflow 119

A river system consists of three main parts in which differ-ent processes dominate: a zone of erosion, a zone of sedimenttransport, and a zone of sediment deposition. It is important torealize that some erosion, transport, and deposition occur in allthree zones; however, within each zone, one of these processesis usually dominant. In addition, the parts of a river system areinterdependent, so that the processes occurring in one part in-fluence the others.

In large river systems erosion is the dominant process in theupstream area, which generally consists of mountainous orhilly topography. Here small tributary streams erode the chan-nels in which they flow and carry material provided by weath-ering and mass wasting.

The region within a river system that is dominated by dep-osition is usually located where the stream enters a large bodyof water. Here sediments accumulate to form a delta, or are re-worked by wave action to form a variety of coastal features. Be-tween the zones of erosion and deposition is the trunk stream

that serves to transport sedi-ments. Taken together, erosion,transportation, and depositionare the processes by whichrivers move Earth’s surface ma-terials and sculpt landscapes.

StreamflowSculpturing Earth’sSurface

� Running Water

Water may flow in one of twoways, either as laminar flow orturbulent flow. In very slow-moving streams the flow is oftenlaminar and the water particlesmove in roughly straight-linepaths that parallel the streamchannel. However, streamflowis usually turbulent, with thewater moving in an erraticfashion that can be charac-terized as a swirling motion.Strong turbulent flow may ex-hibit whirlpools and eddies, aswell as roiling whitewaterrapids. Even streams that ap-pear smooth on the surfaceoften exhibit turbulent flownear the bottom and sides ofthe channel. Turbulence con-tributes to the stream’s abilityto erode its channel because itacts to lift sediment from thestreambed.

Water makes its way to thesea under the influence of

gravity. The time required for the journey depends on the ve-locity of the stream. Velocity is the distance that water trav-els in a unit of time. Some sluggish streams flow at less than

MississippiRiver

DrainageBasin

Yellowstone RiverDrainage

Basin

UpperMississippi

LowerMississippi

MissouriRiver Basin

Yellowstone River

Tongue River

Miles City

Billings

Cody

Powder Rive

r

YellowstoneLake

Tennessee

OhioRiver Basin

Arkansas-Red-White

BBighorn Mts.

Bighorn Mts.

Bighorn R

iver

FIGURE 5.4 A drainage basin is the land area drained by a stream and its tributaries. Divides are the boundaries separating drainage basins. Drainage basins and divides exist for all streams, regardless of size. The drainage basin of the Yellowstone River is one of many that contribute water to the Missouri River, which in turn is one of many that make up the drainage basin of the Mississippi River. The drainage basin of the Mississippi River, North America’s largest, covers about 3 million square kilometers.

Students Sometimes Ask . . .What’s the difference between a stream and a river?

In common usage, these terms imply relative size (a river is largerthan a stream, both of which are larger than a creek or a brook).However, in geology this is not the case: The word stream is usedto denote channelized flow of any size, from a small creek to themightiest river. It is important to note that although the termsriver and stream are sometimes used interchangeably, the termriver is often preferred when describing a main stream into whichseveral tributaries flow.

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120 C H A P T E R 5 Running Water and Groundwater

dients of 10 centimeters perkilometer or less. By contrast,some mountain stream chan-nels decrease in elevation at arate of more than 40 meters perkilometer, or a gradient 400times steeper than the lowerMississippi (Figure 5.6). Gradi-ent varies not only among dif-ferent streams but also over aparticular stream’s length. Thesteeper the gradient, the moreenergy available for stream-flow. If two streams were iden-tical in every respect exceptgradient, the stream with thehigher gradient would obvi-ously have the greater velocity.

A stream’s channel is a con-duit that guides the flow ofwater, but the water encountersfriction as it flows. The shape,size, and roughness of thechannel affect the amount offriction. Larger channels havemore efficient flow because asmaller proportion of water isin contact with the channel. Asmooth channel promotes amore uniform flow, whereas anirregular channel filled withboulders creates enough tur-bulence to slow the stream significantly.

DischargeThe discharge of a stream isthe volume of water flowing

past a certain point in a given unit of time. This is usuallymeasured in cubic meters per second or cubic feet per second.Discharge is determined by multiplying a stream’s cross-sectional area by its velocity:

Table 5.1 lists the world’s largest rivers in terms of dis-charge. The largest river in North America, the Mississippi,discharges an average of 17,300 cubic meters (611,000 cubicfeet) per second. Although this is a huge quantity of water, itis nevertheless dwarfed by the mighty Amazon in SouthAmerica, the world’s largest river. Fed by a vast rainy regionthat is nearly three-fourths the size of the conterminousUnited States, the Amazon discharges 12 times more waterthan the Mississippi.

* velocity 1meters/second2

* channel depth 1meters2

discharge 1m3/second2 = channel width 1meters2

1 kilometer per hour, whereas a few rapid ones may exceed30 kilometers per hour. Velocities are measured at gaging sta-tions (Figure 5.5A). Along straight stretches, the highest ve-locities are near the center of the channel just below thesurface, where friction is lowest (Figure 5.5B). But when astream curves, its zone of maximum speed shifts toward itsouter bank (Figure 5.5C).

The ability of a stream to erode and transport materialsdepends on its velocity. Even slight changes in velocity canlead to significant changes in the load of sediment that watercan transport. Several factors determine the velocity of astream, including (1) gradient; (2) shape, size, and roughnessof the channel; and (3) discharge.

Gradient and Channel CharacteristicsThe slope of a stream channel expressed as the vertical dropof a stream over a specified distance is gradient. Portions ofthe lower Mississippi River, for example, have very low gra-

B.

C.

Maximumvelocity

Conicalcups

Soundingweight

A. Gaging stationMaximumvelocity

FIGURE 5.5 A. Continuous records of stage and discharge are collected by the U.S. Geological Survey at more than 7,000 gauging stations in the United States. Average velocities are determined by usingmeasurements from several spots across the stream. This station is on the Rio Grande south of Taos, New Mexico. (Photo by E. J. Tarbuck) B. Along straight stretches, stream velocity is highest at the center of the channel. C. When a stream curves, its zone of maximum speed shifts toward the outer bank.

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The Work of Running Water 121

The discharges of most rivers are far from constant. This istrue because of such variables as rainfall and snowmelt. Inareas with seasonal variations in precipitation, streamflowwill tend to be highest during the wet season, or during springsnowmelt, and lowest during the dry season or during peri-ods when high temperature increases the water losses throughevapotranspiration. However, not all channels maintain a con-tinuous flow of water. Streams that exhibit flow only during“wet” periods are referred to as intermittent streams. In aridclimates many streams carry water only occasionally after aheavy rainstorm and are called ephemeral streams.

Changes from Upstream to DownstreamOne useful way of studying a stream is to examine its profile.A profile is simply a cross-sectional view of a stream from itssource area (called the head or headwaters) to its mouth, thepoint downstream where the river empties into another

water body. By examining Fig-ure 5.7, you can see that themost obvious feature of a typi-cal profile is a constantly de-creasing gradient from the headto the mouth. Although manylocal irregularities may exist, theoverall profile is a smooth, con-cave, upward curve.

The profile shows that the gra-dient decreases downstream. Tosee how other factors change in adownstream direction, observa-tions and measurements must bemade. When data are collectedfrom several gaging stationsalong a river, they show that in ahumid region discharge increasesfrom the head toward the mouth.This should come as no surprisebecause, as we move down-stream, more and more tributar-ies contribute water to the mainchannel (Figure 5.6). Further-

more, in most humid regions, additional water is added fromthe groundwater supply. Thus, as you move downstream, thestream’s width, depth, and velocity change in response to theincreased volume of water carried by the stream.

Streams that begin in mountainous areas where precipita-tion is abundant and then flow through arid regions may ex-perience the opposite situation. Here discharge may actuallydecrease downstream because of water loss due to evapora-tion, infiltration into the streambed, and removal by irriga-tion. The Colorado River in the southwestern United Statesis such an example.

The Work of Running WaterStreams are Earth’s most important erosional agent. Not onlydo they have the ability to downcut and widen their channelsbut streams also have the capacity to transport the enormous

FIGURE 5.6 Rapids are common in mountain streams where the gradient is steep and the channel is rough and irregular. Although most streamflow is turbulent, it is usually not as rough as that ex-perienced by these river runners at Lost Yak Rapids on Chile’s Rio Bio Bio. (Photo by Carr Clifton)

TABLE 5.1 World’s Largest Rivers Ranked by Discharge

Drainage Area Average Discharge

Rank River Country Square kilometers Square miles Cubic meters per second Cubic feet per second

1 Amazon Brazil 5,778,000 2,231,000 212,400 7,500,0002 Congo Zaire 4,014,500 1,550,000 39,650 1,400,0003 Yangtze China 1,942,500 750,000 21,800 770,0004 Brahmaputra Bangladesh 935,000 361,000 19,800 700,0005 Ganges India 1,059,300 409,000 18,700 660,0006 Yenisei Russia 2,590,000 1,000,000 17,400 614,0007 Mississippi United States 3,222,000 1,244,000 17,300 611,0008 Orinoco Venezuela 880,600 340,000 17,000 600,0009 Lena Russia 2,424,000 936,000 15,500 547,000

10 Parana Argentina 2,305,000 890,000 14,900 526,000

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122 C H A P T E R 5 Running Water and Groundwater

so too can the sand and gravel carried by a stream abrade abedrock channel. Moreover, pebbles caught in swirling ed-dies can act like “drills” and bore circular potholes into thechannel floor (Figure 5.8).

TransportationRegardless of size, all streams transport some rock material.

Streams also sort the solid sediment they transport be-cause finer, lighter material is carried more

rapidly than coarser, heavier rockdebris. Depending on the natureof the rock material, the streamload consists of material (1) insolution (dissolved load), (2) insuspension (suspended load),and (3) sliding or rolling alongthe bottom (bed load).

Dissolved Load Most of thedissolved load is brought to astream by groundwater and is

dispersed throughout the flow.The quantity of material car-ried in solution is highly vari-able and is most abundant inhumid areas where limestone

and other relatively soluble rock forms the bedrock. Usuallythe amount of dissolved load is small and therefore is ex-pressed as parts of dissolved material per million parts ofwater (parts per million, or ppm). Although some rivers mayhave a dissolved load of 1,000 ppm or more, the average fig-ure for the world’s rivers is estimated at 115 to 120 ppm.

quantities of sediment that are delivered to the stream by sheetflow, mass wasting, and groundwater. Eventually much ofthis material is dropped by the water to create a variety of de-positional features.

ErosionA stream’s ability to accumulate and transport soil and weath-ered rock is aided by the work of raindrops, which knocksediment particles loose (see Figure 4.19, p. 100). When theground is saturated, rainwater begins to flow downslope,transporting some of the material it has dislodged. On barrenslopes the sheet flow will often erode small channels, or rills,which in time may evolve into larger gullies (see Figure 4.20on p. 100).

Once the surface flow reaches a stream, its ability to erodeis greatly enhanced by the increase in water volume. Whenthe flow of water is sufficiently strong, it can dislodge parti-cles from the channel and lift them into the moving water. Inthis manner, the force of running water swiftly erodes poorlyconsolidated materials on the bed and sides of a stream chan-nel. On occasion, the banks of the channel may be undercut,dumping even more loose debris into the water to be carrieddownstream.

In addition to eroding unconsolidated materials, the hy-draulic force of streamflow can also cut a channel into solidbedrock. A stream’s ability to erode bedrock is greatly en-hanced by the particles it carries. These particles can be anysize, from large boulders in very fast-flowing waters to sandand gravel-size particles in somewhat slower flow. Just as theparticles of grit on sandpaper can wear away a piece of wood,

Head

Ele

vatio

n (k

m)

0 50 100 150

4

3

2

1

0

Distance (km)

Fresno,CA

Mt. Whitney

Kings River

S i e r r a N e v a d a

Mouth

Longitudinal profile

FIGURE 5.7 A longitudinal profile is a cross-section along the length of a stream. Note the concave-upward curve of the profile, with a steeper gradient upstream and a gentler gradient downstream. Longitudinal profile of California’s Kings River. It originates in the Sierra Nevada and flows westward into the San Joaquin Valley.

FIGURE 5.8 Potholes in the bed of a small stream in Cataract FallsState Park, Indiana. The rotational motion of swirling pebbles acts like adrill to create potholes. (Photo by Tom Till Photography)

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The Work of Running Water 123

The velocity of streamflow has essentially no effect on astream’s ability to carry its dissolved load. Once material is insolution, it goes wherever the stream goes, regardless of ve-locity. Precipitation occurs only when the chemistry of thewater changes.

Suspended Load Most large rivers carry the largest part oftheir load in suspension. Indeed, the visible cloud of sedimentsuspended in the water is the most obvious portion of astream’s load (Figure 5.9). Usually only fine particles consist-ing of silt and clay can be carried this way, but during a flood,sand and even gravel-size particles are transported as well.Also, during a flood, the total quantity of material carried insuspension increases dramatically, as can be verified by anyonewhose home has been a site for the deposition of this material.

The type and amount of material carried in suspension arecontrolled by two factors: the velocity of the water and the set-tling velocity of each sediment grain. Settling velocity is de-fined as the speed at which a particle falls through a still fluid.The larger the particle, the more rapidly it settles toward thestream bed. In addition to size, the shape and specific grav-ity of particles also influence settling velocity. Flat grains sinkthrough water more slowly than do spherical grains, anddense particles fall toward the bottom more rapidly than doless dense particles. The slower the settling velocity and thestronger the turbulence, the longer a sediment particle willstay in suspension, and the farther it will be carried down-stream with the flowing water.

Bed Load A portion of a stream’s load of solid material con-sists of sediment that is too large to be carried in suspension.These coarser particles move along the bottom (bed) of thestream and constitute the bed load.In terms of the erosional work ac-complished by a downcuttingstream, the grinding action of thebed load is of great importance.

The particles that make upthe bed load move along thebottom by rolling, sliding, andsaltation. Sediment moving bysaltation ap-pears to jump or skip along thestream bed. This occurs as par-ticles are propelled upward bycollisions or lifted by the cur-rent and then carried down-stream a short distance untilgravity pulls them back to thebed of the stream. Particles thatare too large or heavy to moveby saltation either roll or slidealong the bottom, dependingon their shapes.

The bed load usually doesnot exceed 10 percent of astream’s total load. Estimates ofa stream’s bed load should be

1saltare = to leap2

viewed cautiously, however, because the transport of sedi-ment as bed load or as suspended load changes frequently.The proportions of each depend on the characteristics ofstreamflow at any time and these may fluctuate over shortintervals. With an increase in velocity, parts of the bed load arethrown into suspension. Conversely, when velocity decreases,a portion of the bed load is deposited and some sediment thathad been suspended changes to moving as bed load.

Competence and Capacity Streams vary in their ability tocarry a load. Their ability is determined by two criteria. First,the competence of a stream measures the maximum size ofparticles it is capable of transporting. The stream’s velocitydetermines its competence. If the velocity of a stream dou-bles, its competence increases four times; if the velocitytriples, its competence increases nine times; and so forth. Thisexplains how large boulders that seem immovable can betransported during a flood, which greatly increases a stream’svelocity.

Second, the capacity of a stream is the maximum load it cancarry. The capacity of a stream is directly related to its dis-charge. The greater the volume of water flowing in a stream,the greater is its capacity for hauling sediment.

By now it should be clear why the greatest erosion andtransportation of sediment occur during floods (Figure 5.9).The increase in discharge results in a greater capacity, andthe increase in velocity results in greater competence. Withrising velocity the water becomes more turbulent, and largerand larger particles are set in motion. In just a few days orperhaps a few hours a stream in flood stage can erode andtransport more sediment than it does during months of nor-mal flow.

FIGURE 5.9 The suspended load is clearly visible because it gives this flooding river a “muddy”appearance. During floods both capacity and competency increase. Therefore, the greatest erosionand sediment transport occur during these high-water periods. The flooding Trinity River neardowntown Dallas, Texas, in late June 2007. The channel in which the river is usually confined isoutlined by trees on either bank. The floodwaters are confined by artificial levees. (Photo by G.J.McCarthy/Dallas Morning News/CORBIS)

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124 C H A P T E R 5 Running Water and Groundwater

when the bed and banks are composed mainly of unconsol-idated sediment, the channel is called an alluvial channel.

Bedrock ChannelsIn their headwaters, where the gradient is usually steepest,many rivers cut into bedrock. Such streams (often mountainstreams) typically transport coarse particles that activelyabrade the bedrock channel. Potholes are often visible evi-dence of the erosional forces at work.

Bedrock channels often alternate between relatively gen-tly sloping segments where alluvium tends to accumulate,and steeper segments where bedrock is exposed. Thesesteeper areas may contain rapids or occasionally a waterfall.The channel pattern exhibited by streams cutting into bedrockis controlled by the underlying geologic structure. Even whenflowing over rather uniform bedrock, streams tend to exhibita winding or irregular pattern rather than flowing in astraight channel. Anyone who has gone on a whitewater raft-ing trip has observed the steep, winding nature of a streamflowing in a bedrock channel.

Alluvial ChannelsMany stream channels are composed of loosely consolidatedsediment (alluvium) and therefore can undergo major changesin shape because the sediments are continually being eroded,transported, and redeposited. The major factors affecting theshapes of these channels is the average size of the sediment

DepositionWhenever a stream slows down, the situation reverses. Asits velocity decreases, its competence is reduced and sedi-ment begins to drop out, largest particles first. Recall thateach particle size has a settling velocity. As streamflow dropsbelow the setting velocity of a certain particle size, sedimentin that category begins to settle out. Thus, stream transportprovides a mechanism by which solid particles of varioussizes are separated. This process, called sorting, explains whyparticles of similar size are deposited together.

The material deposited by a stream is called alluvium, thegeneral term for any stream-deposited sediment. Many dif-ferent depositional features are composed of alluvium. Someoccur within stream channels, some occur on the valley flooradjacent to the channel, and some exist at the mouth of thestream. We will consider the nature of these features later.

Stream ChannelsA basic characteristic of streamflow that distinguishes it fromsheet flow is that it is usually confined to a channel. A streamchannel can be thought of as an open conduit that consists ofthe streambed and banks that act to confine the flow exceptduring floods.

Although somewhat oversimplified, we can divide streamchannels into two types. Bedrock channels are those in whichthe streams are actively cutting into solid rock. In contrast,

Depositionof point bar

Erosion of cut bank

Maximumvelocity

Cut bank inMarch 1965

Point bar

Cut bank in January 1965

FIGURE 5.10 When a stream meanders, its zone of maximum speed shifts toward the outer bank.A point bar is deposited where the water on the inside of a meander slows. The point bar shownhere is on the Missouri River in North Dakota. The black and white photos show erosion of a cutbank along the Newaukum River in Washington State. By eroding the outer bank and depositingmaterial on the inside of the bend, a stream is able to shift its channel. (Point bar photo by CarrClifton; cut bank photos by P. A. Glancy, U.S. Geological Survey)

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Stream Channels 125

being transported, the channelgradient, and the discharge.

Alluvial channel patterns re-flect a stream’s ability to trans-port its load at a uniform rate,while expending the leastamount of energy. Thus, the sizeand type of sediment being car-ried help determine the natureof the stream channel. Two com-mon types of alluvial channelsare meandering channels andbraided channels.

Meandering Streams Streams thattransport much of their load in sus-pension generally move in sweepingbends called meanders. These streamsflow in relatively deep, smooth channelsand transport mainly mud (silt and clay).The lower Mississippi River exhibits a chan-nel of this type.

Because of the cohesiveness of consolidatedmud, the banks of stream channels carrying fineparticles tend to resist erosion. As a consequence,most of the erosion in such channels occurs on theoutside of the meander, where velocity and turbu-lence are greatest. In time, the outside bank is under-mined, especially during periods of high water. Becausethe outside of a meander is a zone of active erosion, it isoften referred to as the cut bank (Figure 5.10). The debrisacquired by the stream at the cut bank moves downstreamwith the coarser material generally being deposited aspoint bars in zones of decreased velocity on the insides ofmeanders. In this manner, meanders migrate laterally byeroding the outside of the bends and depositing on the inside.

In addition to migrating laterally, the bends in a channelalso migrate down the valley. This occurs because erosion ismore effective on the downstream (downslope) side of themeander. Sometimes the downstream migration of a mean-der is slowed when it reaches a more resistant material. Thisallows the next meander upstream to “catch up” and overtakeit, as shown in Figure 5.11. Gradually the neck of land be-tween the meanders is narrowed. Eventually, the river mayerode through the narrow neck of land to the next loop(Figure 5.11). The new, shorter channel segment is called acutoff and, because of its shape, the abandoned bend is calledan oxbow lake (Figure 5.12).

Braided Streams Some streams consist of a complex net-work of converging and diverging channels that thread theirway among numerous islands or gravel bars (Figure 5.13).Because these channels have an interwoven appearance, thesestreams are said to be braided. Braided channels form wherea large proportion of the stream’s load consists of coarse ma-terial (sand and gravel) and the stream has a highly variable

Neck

TI

ME

Oxbow lake

Plugs withsilt and clay

FIGURE 5.11 Formation of a cutoff and oxbow lake.

FIGURE 5.12 Oxbow lakes occupy abandoned meanders. As they fillwith sediment, oxbow lakes gradually become swampy meander scars.Aerial view of an oxbow lake created by the meandering Green River nearBronx, Wyoming. (Photo by Michael Collier)

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lowest elevation to which a stream can erode its channel. Es-sentially this is the level at which the mouth of a stream en-ters the ocean, a lake, or another stream. Base level accountsfor the fact that most stream profiles have low gradients neartheir mouths, because the streams are approaching the ele-vation below which they cannot erode their beds.

Two general types of base level are recognized. Sea level isconsidered the ultimate base level, because it is the lowest levelto which stream erosion could lower the land. Temporary, orlocal, base levels include lakes, resistant layers of rock, andmain streams that act as base levels for their tributaries. Forexample, when a stream enters a lake, its velocity quickly ap-proaches zero and its ability to erode ceases. Thus, the lakeprevents the stream from eroding below its level at any pointupstream from the lake. However, because the outlet of thelake can cut downward and drain the lake, the lake is only atemporary hindrance to the stream’s ability to downcut itschannel. In a similar manner, the layer of resistant rock at thelip of the waterfall in Figure 5.14 acts as a temporary baselevel. Until the ledge of hard rock is eliminated, it will limitthe amount of downcutting upstream.

Any change in base level will cause a corresponding read-justment of stream activities. When a dam is built along astream, the reservoir that forms behind it raises the base levelof the stream (Figure 5.15). Upstream from the dam the gra-dient is reduced, lowering the stream’s velocity and, hence,its sediment-transporting ability. The stream, now having toolittle energy to transport its entire load, will deposit sediment.This builds up its channel. Deposition will be the dominantprocess until the stream’s gradient increases sufficiently totransport its load.

discharge. Because the bank material is readily erodable,braided channels are wide and shallow.

One circumstance in which braided streams form is at theend of a glacier where there is a large seasonal variation indischarge. Here, large amounts of ice-eroded sediment aredumped into the meltwater streams flowing away from theglacier. When flow is sluggish, the stream is unable to moveall of the sediment and therefore deposits the coarsest mate-rial as bars that force the flow to split and follow severalpaths. Usually the laterally shifting channels completely re-work most of the surface sediments each year, thereby trans-forming the entire streambed. In some braided streams,however, the bars have built up to form islands that are an-chored by vegetation.

In summary, meanderingchannels develop where theload consists largely of fine-grained particles that are trans-ported as suspended load in adeep, smooth channel. By con-trast, wide, shallow braidedchannels develop where coarse-grained alluvium is transportedas bedload.

Base Level andStream ErosionStreams cannot endlesslyerode their channels deeperand deeper. There is a lowerlimit to how deep a stream canerode, and that limit is calledbase level. Although the ideais relatively straightforward, itis nevertheless a key conceptin the study of stream activity.Base level is defined as the

FIGURE 5.13 Braided stream choked with sediment near the terminusof a melting glacier. (Photo by Bradford Washburn)

Profile of streamif resistant rock

did not exist

Resistant bed

Local baselevel

Waterfalls

Ultimate base level

A.

B.

C.

Local baselevel

Rapids

Resistant bed

Sea

Profile of streamadjusted to

ultimate base level

Sea

Resistant bedSea

Ultimate base level

Ultimate base level

FIGURE 5.14 A resistant layer of rock can act as a local (temporary) base level. Because thedurable layer is eroded more slowly, it limits the amount of downcutting upstream.

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Shaping Stream Valleys 127

Shaping Stream ValleysSculpturing Earth’s Surface� Running Water

Streams, with the aid of weathering and mass wasting, shapethe landscape through which they flow. As a result, streamscontinuously modify the valleys that they occupy.

A stream valley consists not only of the channel but alsothe surrounding terrain that directly contributes water to thestream. Thus it includes the valley bottom, which is the lower,flatter area that is partially or totally occupied by the streamchannel, and the sloping valley walls that rise above the val-ley bottom on both sides. Most stream valleys are muchbroader at the top than is the width of their channel at thebottom. This would not be the case if the only agent respon-sible for eroding valleys were the streams flowing throughthem. The sides of most valleys are shaped by a combinationof weathering, overland flow, and mass wasting. In some aridregions, where weathering is slow and where rock is partic-ularly resistant, narrow valleys having nearly vertical wallsare common.

Stream valleys can be divided into two general types—narrow, V-shaped valleys and wide valleys with flat floors—with many gradations between.

Valley DeepeningWhen a stream’s gradient is steep and the channel is wellabove base level, downcutting is the dominant activity. Abra-sion caused by bed load sliding and rolling along the bottom,and the hydraulic power of fast-moving water, slowly lowersthe streambed. The result is usually a V-shaped valley with

steep sides. A classic exampleof a V-shaped valley is locatedin the section of YellowstoneRiver shown in Figure 5.16.

The most prominent fea-tures of a V-shaped valley arerapids and waterfalls. Both oc-cur where the stream’s gradi-ent increases significantly, asituation usually caused byvariations in the erodabilityof the bedrock into which astream channel is cutting. Re-sistant beds create rapids byacting as a temporary baselevel upstream while allow-ing downcutting to continuedownstream. In time erosionusually eliminates the resist-ant rock. Waterfalls are placeswhere the stream makes anabrupt vertical drop.

Dam

Original profile

New stream profileformed by deposition

of sediment

Sea

Sea

Ultimatebase level

Profile of streamadjusted tobase level

Reservoir

Newbase level

A.

B.

FIGURE 5.15 When a dam is built and a reservoir forms, the stream’s base level is raised. This reduces the stream’s velocity and leads to deposition and a reduction of the gradient upstream from the reservoir.

FIGURE 5.16 V-shaped valley of the Yellowstone River. The rapids andwaterfalls indicate that the river is vigorously downcutting. (Photo by ArtWolfe)

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128 C H A P T E R 5 Running Water and Groundwater

Changing Base Level and Incised MeandersWe usually expect a stream with a highly meandering course tobe on a floodplain in a wide valley. However, certain rivers ex-hibit meandering channels that flow in steep, narrow valleys.Such meanders are called incisedmeanders (Figure 5.18). How do such features form?

Originally, the meandersprobably developed on thefloodplain of a stream thatwas relatively near base level.Then, a change in base levelcaused the stream to begindowncutting. One of twoevents could have occurred.Either base level dropped orthe land upon which the riverwas flowing was uplifted.

An example of the first cir-cumstance happened duringthe Ice Age when large quanti-ties of water were withdrawnfrom the ocean and locked upin glaciers on land. The resultwas that sea level (ultimatebase level) dropped, causingrivers flowing into the ocean tobegin to downcut. Of course,this activity ceased at the closeof the Ice Age when ice sheetsmelted and sea level rose.

Regional uplift of the land,the second cause for incisedmeanders, is exemplified bythe Colorado Plateau in thesouthwestern United States.

1incisum = to cut into2

Valley WideningOnce a stream has cut its channel closer to base level, down-ward erosion becomes less dominant. At this point thestream’s channel takes on a meandering pattern, and moreof the stream’s energy is directed from side to side. The resultis a widening of the valley as the river cuts away first at onebank and then at the other (Figure 5.17). The continuous lat-eral erosion caused by shifting of the stream’s meanders pro-duces an increasingly broader, flat valley floor covered withalluvium. This feature, called a floodplain, is appropriatelynamed because when a river overflows its banks during floodstage, it inundates the floodplain.

Over time the floodplain will widen to the point that thestream is only actively eroding the valley walls in a fewplaces. In fact, in large rivers such as the lower MississippiRiver valley, the distance from one valley wall to another canexceed 100 miles.

Students Sometimes Ask . . .What’s the highest waterfall in the world?

The world’s highest uninterrupted waterfall is Angel Falls onVenezuela’s Churun River. Named for American aviator JimmieAngel, who first sighted the falls from the air in 1933, the riverplunges 979 meters (3,212 feet, or more than 0.6 mile).

NarrowV-shaped valley

Floodplainwell developed

A.

B.

C.

Site of erosion

TI

ME

Site of deposition

FIGURE 5.17 Stream eroding its floodplain.

FIGURE 5.18 Incised meanders of the Colorado River in CanyonlandsNational Park, Utah. Here, as the Colorado Plateau was gradually uplifted,the meandering river adjusted to being higher above base level bydowncutting. (Photo by Michael Collier)

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Depositional Landforms 129

Here, as the plateau was gradually uplifted, numerous me-andering rivers adjusted to being higher above base level bydowncutting (Figure 5.18).

Depositional LandformsAs indicated earlier, whenever a stream’s velocity slows, itbegins to deposit some of the sediment it is carrying. Also re-call that streams continually pick up sediment in one part oftheir channel and redeposit it downstream. These channeldeposits are most often composed of sand and gravel and arecommonly referred to as bars. Such features, however, areonly temporary, for the material will be picked up again andeventually carried to the ocean. In addition to sand and gravelbars, streams also create other depositional features that havea somewhat longer life span. These include deltas, naturallevees, and alluvial fans.

DeltasWhen a stream enters the relatively still waters of an ocean orlake, its velocity drops abruptly, and the resulting depositsform a delta (Figure 5.19). As the delta grows outward, thestream’s gradient continually lessens. This circumstance even-tually causes the channel to become choked with sedimentdeposited from the slowing water. As a consequence, the riverseeks a shorter, higher-gradient route to base level, as illus-trated in Figure 5.19B. This illustration shows the main chan-nel dividing into several smaller ones, called distributaries.Most deltas are characterized by these shifting channels thatact in an opposite way to that of tributaries.

Rather than carrying water into the main channel, dis-tributaries carry water away from the main channel. Afternumerous shifts of the channel, a delta may grow into arough triangular shape like the Greek letter delta forwhich it is named. Note, however, that many deltas do notexhibit the idealized shape. Differences in the configura-tions of shorelines and variations in the nature and strengthof wave activity result in many shapes. Many large rivershave deltas extending over thousands of square kilometers.The delta of the Mississippi River is one example (see Box5.1). It resulted from the accumulation of huge quantities ofsediment derived from the vast region drained by the riverand its tributaries (see Figure 5.4, p. 119). Today, New Or-leans rests where there was ocean less than 5,000 years ago.Figure 5.20 shows that portion of the Mississippi delta thathas been built over the past 5,000 to 6,000 years. As you cansee, the delta is actually a series of seven coalescing sub-deltas. Each formed when the river left its existing channelin favor of a shorter, more direct path to the Gulf of Mexico.The individual subdeltas interfinger and partially cover oneanother to produce a very complex structure. The presentsubdelta, called a bird-foot delta because of the configura-tion of its distributaries, has been built by the Mississippiin the last 500 years.

Natural LeveesSome rivers occupy valleys with broad floodplains and buildnatural levees that parallel their channels on both banks(Figure 5.21). Natural levees are built by successive floods overmany years. When a stream overflows its banks, its velocityimmediately diminishes, leaving coarse sediment deposited

in strips bordering the channel.As the water spreads out overthe valley, a lesser amount offine sediment is deposited overthe valley floor. This unevendistribution of material pro-duces the very gentle slope ofthe natural levee.

The natural levees of thelower Mississippi rise 6 meters(20 feet) above the floodplain.The area behind the levee ischaracteristicallypoorlydrainedfor theobviousreasonthatwatercannot flow up the levee andinto the river. Marshes calledbackswamps result.Atributarystream that cannot enter a riverbecause levees block the wayoften has to flow parallel to theriveruntil it canbreachthe levee.Such streams are called yazootributaries after theYazooRiver,which parallels the Mississippifor over 300 kilometers (about190 miles).

1¢2,

A.

Topset beds

Distributaries

Lake

Foreset beds

Bottomset beds

B.

FIGURE 5.19 A. Structure of a simple delta that forms in the relatively quiet waters of a lake. B. Growth of a simple delta. As a stream extends its channel, the gradient is reduced. Frequently, during flood stage the river is diverted to a higher-gradient route, forming a new distributary. Old, abandoned distributaries are gradually invaded by aquatic vegetation and fill with sediment.(After Ward’s Natural Science Establishment, Inc., Rochester, N.Y.)

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130 C H A P T E R 5 Running Water and Groundwater

Baton Rouge

New Orleans

Gulf of Mexico

1

2

3

4

5

6

7

Atchafalaya River

2

7

4

FIGURE 5.20 During the past 5,000 to 6,000 years, the Mississippi River has built a series of sevencoalescing subdeltas. The numbers indicate the order in which the subdeltas were deposited. Thepresent birdfoot delta (number 7) represents the activity of the past 500 years. (Image courtesy ofJPL/Cal Tech/NASA) Without ongoing human efforts, the present course will shift and follow the pathof the Atchafalaya River. The inset on the left shows the point where the Mississippi may somedaybreak through (arrow) and the shorter path it would take to the Gulf of Mexico. (After C. R. Kolb andJ. R. Van Lopik)

Valleywall

Coarse sedimentsFine sediments

Yazoo tributary

Naturallevees

Floodplain

Back swamp

FloodplainCoarse sediments

depositedFine sediments

deposited

Developingnatural levee

Natural levee

Natural levee after numerous floods

Floodstage

Post flood

Floodstage

Coarse sedimentsdeposited

Fine sedimentsdeposited

FIGURE 5.21 Natural levees are gently sloping deposits that are created by repeated floods. Thediagrams on the right show the sequence of development. Because the ground next to the streamchannel is higher than the adjacent floodplain, back swamps and yazoo tributaries may develop.

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Depositional Landforms 131

BOX 5.1 � PEOPLE AND THE ENVIRONMENT

Coastal Wetlands Are Vanishing on the Mississippi DeltaCoastal wetlands form in sheltered envi-ronments that include swamps, tidal flats,coastal marshes, and bayous. They are richin wildlife and provide nesting grounds andimportant stopovers for waterfowl and mi-gratory birds, as well as spawning areas andvaluable habitats for fish.

The delta of the Mississippi River inLouisiana contains about 40 percent of allcoastal wetlands in the lower 48 states.Louisiana’s wetlands are sheltered from thewave action of hurricanes and winterstorms by low-lying offshore barrier islands.Both the wetlands and the barrier islandshave formed as a result of the shifting of theMississippi River during the past 7,000years.

The dependence of Louisiana’s coastalwetlands and offshore islands on the Mis-sissippi River and its distributaries as a direct source of sediment leaves them vul-nerable to changes in the river system.Moreover, the reliance on barrier islands forprotection from storm waves leaves coastalwetlands vulnerable when these narrow off-shore islands are eroded.

Today, the coastal wetlands of Louisianaare disappearing at an alarming rate. Al-though Louisiana contains 40 percent of thewetlands in the lower 48 states, it accountsfor 80 percent of the wetland loss. Accord-

ing to the U.S. Geological Survey, Louisianalost nearly 5,000 square kilometers (1,900square miles) of coastal land between 1932and 2000. The state continues to lose be-tween 65 and 91 square kilometers (25 to 35square miles) each year. At this rate another1,800 to 4,500 square kilometers (700 to 1,750square miles) will vanish under the Gulf ofMexico by the year 2050.* Global climatechange could increase the severity of theproblem because rising sea level andstronger tropical storms accelerate rates ofcoastal erosion.** Unfortunately, this wasobserved firsthand during the extraordinary2005 hurricane season when hurricanes Ka-trina and Rita devastated portions of theGulf Coast.

By nature, the delta, its wetlands, and theadjacent barrier islands are dynamic fea-tures. Over the millennia, as sediment ac-cumulated and built the delta in one area,erosion and subsidence caused losses else-where. Whenever the river shifted, thezones of delta growth and destruction alsoshifted. However, with the arrival of peo-ple, this relative balance between formationand destruction changed—the rate at whichthe delta and its wetlands were destroyedaccelerated and now greatly exceeds therate of formation. Why are Louisiana’s wet-lands shrinking?

Before Europeans settled the delta, theMississippi River regularly overflowed itsbanks in seasonal floods. The huge quanti-ties of sediment that were deposited re-newed the soil and kept the delta fromsinking below sea level. However, with set-tlement came flood-control efforts and thedesire to maintain and improve navigationon the river. Artificial levees were con-structed to contain the rising river duringflood stage. Over time the levees were extended all the way to the mouth of theMississippi to keep the channel open fornavigation.

The effects have been straightforward.The levees prevent sediment and freshwater from being dispersed into the wet-lands. Instead, the river is forced to carry itsload to the deep waters at the mouth. Mean-while, the processes of compaction, subsi-dence, and wave erosion continue. Becausenot enough sediment is added to offsetthese forces, the size of the delta and the ex-tent of its wetlands gradually shrink.

The problem has been aggravated by adecline in the sediment transported by theMississippi, decreasing by approximately50 percent over the past 100 years. A sub-stantial portion of the reduction results from trapping of sediment in large reser-voirs created by dams built on tributariesto the Mississippi.

Another factor contributing to wetlanddecline is the fact that the delta is laced with13,000 kilometers (8,000 miles) of navigationchannels and canals. These artificial open-ings to the sea allow salty Gulf waters to flowfar inland. The invasion of saltwater andtidal action causes massive “brownouts” ormarsh die-offs (Figure 5.A).

Understanding and modifying the im-pact of people is a necessary basis for anyplan to reduce the loss of wetlands in theMississippi delta. The U.S. Geological Sur-vey estimates that restoring Louisiana’scoasts will require about $14 billion over thenext 40 years. What if nothing is done? Stateand federal officials estimate that costs ofinaction could exceed $100 billion.

*See “Louisiana’s Vanishing Wetlands: Going, Going . . .”in Science, Vol. 289, 15 September 2000, pp. 1860–63. Alsosee Elizabeth Kolbert, “Watermark—Can SouthernLouisiana, be Saved?” The New Yorker, February 27, 2006,pp. 46–57.

**“For more on this possibility, see “Some Possible Con-sequences of Global Warming” in Chapter 20.

FIGURE 5.A This group of dead cypress trees, known as a ghost forest, was killed by encroachingsalt water in Terrebonne Parish, Louisiana. (Photo by Robert Caputo/Aurora Photos)

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132 C H A P T E R 5 Running Water and Groundwater

like.” The dendritic patternforms where the underlyingmaterial is relatively uniform.Because the surface material isessentially uniform in its re-sistance to erosion, it does notcontrol the pattern of stream-flow. Rather, the pattern is determined chiefly by the di-rection of slope of the land.

When streams diverge froma central area like spokes fromthe hub of a wheel, the patternis said to be radial (Figure5.22B). This pattern typicallydevelops on isolated volcaniccones and domal uplifts.

Figure 5.22C illustrates arectangular pattern, in whichmany right-angle bends can beseen. This pattern developswhen the bedrock is criss-crossed by a series of jointsand/or faults. Because thesestructures are eroded moreeasily than unbroken rock,their geometric pattern guidesthe directions of valleys.

Figure 5.22D illustrates a trellis drainage pattern, a rec-tangular pattern in which tributary streams are nearly paral-lel to one another and have the appearance of a garden trellis.This pattern forms in areas underlain by alternating bandsof resistant and less-resistant rock.

Floods and Flood ControlWhen the discharge of a stream becomes so great that it ex-ceeds the capacity of its channel, it overflows its banks as aflood. Floods are among the most deadly and most destruc-tive of all geologic hazards. They are, nevertheless, simplypart of the natural behavior of streams.

Causes of FloodsRivers flood because of the weather. Rapid melting of snowin the spring and/or major storms that bring heavy rains overa large region cause most floods. The extensive 1997 floodalong the Red River of the north is a recent example of anevent triggered by rapid snowmelt. Exceptional rains causedthe devastating floods in the upper Mississippi River Valleyduring the summer of 1993 (Figure 5.23).

Unlike the extensive regional floods just mentioned, flashfloods are more limited in extent. Flash floods occur with lit-tle warning and can be deadly because they produce a rapidrise in water levels and can have a devastating flow velocity.Several factors influence flash flooding. Among them are rainfall intensity and duration, topography, and surface

Alluvial FansAlluvial fans typically develop where a high-gradient streamleaves a narrow valley in mountainous terrain and comes outsuddenly onto a broad, flat plain or valley floor (see Figure6.30, p. 177). Alluvial fans form in response to the abrupt dropin gradient combined with the change from a narrow chan-nel of a mountain stream to less confined channels at the baseof the mountains. The sudden drop in velocity causes thestream to dump its load of sediment quickly in a distinctivecone- or fan-shaped accumulation. As illustrated by Figure6.28, the surface of the fan slopes outward in a broad arc froman apex at the mouth of the steep valley. Usually, coarse ma-terial is dropped near the apex of the fan, while fine materialis carried toward the base of the deposit.

Drainage PatternsSculpturing Earth’s Surface� Running Water

Drainage systems are networks of streams that together formdistinctive patterns. The nature of a drainage pattern can varygreatly from one type of terrain to another, primarily in re-sponse to the kinds of rock on which the streams developedor the structural pattern of faults and folds.

The most commonly encountered drainage pattern is thedendritic pattern (Figure 5.22A). This pattern of irregularlybranching tributary streams resembles the branching patternof a deciduous tree. In fact, the word dendritic means “tree-

A. Dendritic

Volcano

B. Radial

C. RectangularD. Trellis

Ridges ofresistant

rock

Valleys cut inless-resistant

rock

FIGURE 5.22 Drainage patterns. A. Dendritic. B. Radial. C. Rectangular. D. Trellis.

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Floods and Flood Control 133

July 4, 1988

Mississippi River

Missouri River

July 18, 1993

Mississippi River

Missouri River

FIGURE 5.23 Satellite views of the Missouri River flowing into theMississippi River. St. Louis is just south of their confluence. The upperimage shows the rivers during a drought that occurred in summer 1988.The lower image depicts the peak of the record-breaking 1993 flood.Exceptional rains produced the wettest spring and early summer of thetwentieth century in the upper Mississippi River basin. In all, nearly 14million acres were inundated, displacing at least 50,000 people. (Photoscourtesy of GeoEye.www.geoeye.com. © 2006. All rights reserved.)

conditions. Mountainous areas are susceptible because steepslopes can quickly funnel runoff into narrow canyons. Urbanareas are susceptible to flash floods because a high percent-age of the surface area is composed of impervious surfacessuch as roofs, streets, and parking lots where runoff is veryrapid. In fact, a recent study indicated that the area of im-pervious surfaces in the United States (excluding Alaska andHawaii) amounts to more than 112,600 square kilometers(nearly 44,000 square miles), which is slightly less than thearea of the state of Ohio.*

Human interference with the stream system can worsenor even cause floods. A prime example is the failure of a damor an artificial levee. These structures are built for flood pro-tection. They are designed to contain floods of a certain mag-

nitude. If a larger flood occurs, the dam or levee is over-topped. If the dam or levee fails or is washed out, the waterbehind it is released to become a flash flood. The bursting ofa dam in 1889 on the Little Conemaugh River caused the dev-astating Johnstown, Pennsylvania, flood that took some 3,000lives. A second dam failure occurred there again in 1977 andcaused 77 fatalities.

Flood ControlSeveral strategies have been devised to eliminate or lessenthe catastrophic effects of floods. Engineering efforts includethe construction of artificial levees, the building of flood-control dams, and river channelization.

Artificial Levees Artificial levees are earthen mounds built onthe banks of a river to increase the volume of water the chan-nel can hold. These most common of stream-containmentstructures have been used since ancient times and continue tobe used today.

Artificial levees are usually easy to distinguish from nat-ural levees because their slopes are much steeper. In some lo-cations, especially urban areas, concrete floodwalls aresometimes constructed that serve the same purpose as artifi-cial levees.

Such structures do not always provide the flood protec-tion that was intended. Many artificial levees were not builtto withstand periods of extreme flooding. For example, leveefailures were numerous in the Midwest during the summerof 1993, when the upper Mississippi and many of its tribu-taries experienced record floods (Figure 5.24). During thatsame event, floodwalls at St. Louis, Missouri, created a bot-tleneck for the river that led to increased flooding upstreamof the city.

Flood-Control Dams Flood-control dams are built to storefloodwater and then let it out slowly. This lowers the floodcrest by spreading it out over a longer time span. Since the1920s, thousands of dams have been built on nearly every

Students Sometimes Ask. . .Sometimes when there is a major flood, it is described as a100-year flood. What does that mean?

The phrase “100-year flood” is misleading because it leads peo-ple to believe that such an event happens only once every 100years. The truth is that an uncommonly big flood can happenany year. The phrase “100-year flood” is really a statistical des-ignation, indicating that there is a 1-in-100 chance that a floodthis size will happen during any year. Perhaps a better termwould be the “1-in-100-chance flood.”

Many flood designations are reevaluated and changed overtime as more data are collected or when a river basin is alteredin a way that affects the flow of water. Dams and urban devel-opment are examples of some human influences in a basin thataffect floods.*C. D. Elvidge, et al. “U.S. Constructed Area Approaches the Size of Ohio,” in EOS,

Transactions, American Geophysical Union, Vol. 85, No. 24, 15 June 2004, p. 233.

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ened more than 240 kilometers (150 miles). The program hasbeen somewhat successful in reducing the height of the riverin flood stage. However, because the river’s tendency towardmeandering still exists, preventing the river from returning toits previous course has been difficult.

A Nonstructural Approach All of the flood-control meas-ures described so far have involved structural solutionsaimed at “controlling” a river. These solutions are expensiveand often give people residing on the floodplain a false senseof security.

Today, many scientists and engineers advocate a non-structural approach to flood control. They suggest that an al-ternative to artificial levees, dams, and channelization issound floodplain management. By identifying high-riskareas, appropriate zoning regulations can be implementedto minimize development and promote more appropriateland use.

Groundwater: Water Beneath the Surface

Sculpturing Earth’s Surface� Groundwater

Groundwater is one of our most important and widely avail-able resources. Yet people’s perceptions of groundwater areoften unclear and incorrect. The reason is that groundwateris hidden from view except in caves and mines, and the im-pressions people gain from these subsurface openings areoften misleading. Observations on the land surface give animpression that Earth is solid. This view is not changed very

major river in the United States. Many dams have significantnonflood-related functions, such as providing water for irri-gated agriculture and for hydroelectric power generation.Many reservoirs are also major regional recreational facilities.

Although dams may reduce flooding and provide otherbenefits, building these structures also has significant costsand consequences. For example, reservoirs created by damsmay cover fertile farmland, useful forests, historic sites, andscenic valleys. Of course, dams trap sediment. Therefore,deltas and floodplains downstream erode because they areno longer replenished with silt during floods (see Box 5.1).Large dams can also cause significant ecological damage toriver environments that took thousands of years to establish.

Building a dam is not a permanent solution to flooding.Sedimentation behind a dam means that the volume of itsreservoir will gradually diminish, reducing the effectivenessof this flood-control measure.

Channelization Channelization involves altering a streamchannel in order to speed the flow of water to prevent it fromreaching flood height. This may simply involve clearing achannel of obstructions or dredging a channel to make itwider and deeper.

Another alteration involves straightening a channel by cre-ating artificial cutoffs. The idea is that by shortening the stream,the gradient and hence the velocity are both increased. By in-creasing velocity, the larger discharge associated with flood-ing can be dispersed more rapidly.

Since the early 1930s, the U.S. Army Corps of Engineershas created many artificial cutoffs on the Mississippi for thepurpose of increasing the efficiency of the channel and re-ducing the threat of flooding. In all, the river has been short-

River

Break in Levee

FIGURE 5.24 Water rushes through a break in an artificial levee in Monroe County, Illinois. Duringthe record-breaking 1993 Midwest floods, many artificial levees could not withstand the force of thefloodwaters. Sections of many weakened structures were overtopped or simply collapsed. (Photo byJames A. Finley/AP/Wide World Photos)

134 C H A P T E R 5 Running Water and Groundwater

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Groundwater: Water Beneath the Surface 135

much when we enter a cave and see water flowing in a chan-nel that appears to have been cut into solid rock. Because ofsuch observations, many people believe that groundwateroccurs only in underground “rivers.” But actual rivers un-derground are extremely rare.

In reality, most of the subsurface environment is not solidat all. It includes countless tiny pore spaces between grains ofsoil and sediment, plus narrow joints and fractures in bedrock.Together, these spaces add up to an immense volume. It is inthese tiny openings that groundwater collects and moves.

The Importance of GroundwaterConsidering the entire hydrosphere, or all of Earth’s water, onlyabout six-tenths of 1 percent occurs underground. Neverthe-less, this small percentage, stored in the rocks and sediments be-neath Earth’s surface, is a vast quantity. When the oceans areexcluded and only sources of freshwater are considered, thesignificance of groundwater becomes more apparent.

Table 5.2 contains estimates of the distribution of freshwaterin the hydrosphere. Clearly, the largest volume occurs as glacialice. Second in rank is groundwater, with slightly more than 14percent of the total. However, when ice is excluded and just liq-uid water is considered, more than 94 percent is groundwater.Without question, groundwater represents the largest reservoir offreshwater that is readily available to humans. Its value in terms ofeconomics and human well-being is incalculable.

Worldwide, wells and springs provide water for cities,crops, livestock, and industry. In the United States, ground-water is the source of about 40 percent of the water used forall purposes (except hydroelectric power generation andpower-plant cooling). Groundwater is the drinking water formore than 50 percent of the population, is 40 percent of thewater for irrigation, and provides more than 25 percent of in-dustry’s needs. In some areas, however, overuse of this basicresource has caused serious problems, including streamflowdepletion, land subsidence, and increased pumping costs. Inaddition, groundwater contamination due to human activitiesis a real and growing threat in many places.

Groundwater’s Geological RolesGeologically, groundwater is important as an erosional agent.The dissolving action of groundwater slowly removes solublerock, allowing surface depressions known as sinkholes to form

TABLE 5.2 Fresh Water of the Hydrosphere

Parts of the Hydrosphere

Volume ofFresh Water1km32

Share of Total Volume of Fresh Water (percent)

Ice sheets and glaciers 24,000,000 84.945Groundwater 4,000,000 14.158Lakes and reservoirs 155,000 0.549Soil moisture 83,000 0.294Water vapor in the atmosphere 14,000 0.049River water 1,200 0.004Total 28,253,200 100.00Source: U.S. Geological Survey Water Supply Paper 2220, 1987.

A.

B.

FIGURE 5.25 A. A view of the interior of Three Fingers Cave, LincolnCounty, New Mexico. The dissolving action of groundwater created thecavern. Later, groundwater deposited the limestone decorations. (Photo© David Muench) B. Groundwater was responsible for creating thesesinkholes in a limestone plateau north of Jajce, Bosnia and Herzegovina.(Photo by Jerome Wyckoff/Animals Animals/Earth Scenes)

as well as creating subterranean caverns (Figure 5.25).Groundwater is also an equalizer of streamflow. Much of thewater that flows in rivers is not direct runoff from rain andsnowmelt. Rather, a large percentage of precipitation soaks inand then moves slowly underground to stream channels.Groundwater is thus a form of storage that sustains streams

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136 C H A P T E R 5 Running Water and Groundwater

during periods when rain does not fall. When we observewater flowing in a river during a dry period, it is water fromrain that fell at some earlier time and was stored underground.

Distribution and Movement of Groundwater

Sculpturing Earth’s Surface� Groundwater

When rain falls, some of the water runs off, some returns tothe atmosphere by evaporation and transpiration, and the re-mainder soaks into the ground. This last path is the primarysource of practically all subsurface water. The amount ofwater that takes each of these paths, however, varies consid-erably both in time and space. Influential factors include thesteepness of the slopes, the nature of the surface materials,the intensity of the rainfall,and the type and amount ofvegetation. Heavy rains fallingon steep slopes underlain byimpervious materials will ob-viously result in a high per-centage of the water runningoff. Conversely, if rain fallssteadily and gently on moregradual slopes composed ofmaterials that are easily pene-trated by water, a much largerpercentage of the water soaksinto the ground.

DistributionSome of the water that soaksin does not travel far, becauseit is held by molecular attrac-tion as a surface film on soilparticles. This near-surfacezone is called the zone of soilmoisture. It is crisscrossed byroots, voids left by decayed

roots, and animal and worm burrows that enhance the infil-tration of rainwater into the soil. Soil water is used by plantsin life functions and transpiration. Some water also evapo-rates directly back into the atmosphere.

Water that is not held as soil moisture percolates down-ward until it reaches a zone where all of the open spaces insediment and rock are completely filled with water. This is thezone of saturation. Water within it is called groundwater.The upper limit of this zone is known as the water table. Thearea above the water table where the soil, sediment, and rockare not saturated is called the unsaturated zone (Figure 5.26).Although a considerable amount of water can be present inthe unsaturated zone, this water cannot be pumped by wells,because it clings too tightly to rock and soil particles. By con-trast, below the water table, the water pressure is greatenough to allow water to enter wells, thus permitting ground-water to be withdrawn for use. We examine wells moreclosely later in the chapter.

The water table is rarely level as we might expect a tableto be. Instead, its shape is usually a subdued replica of thesurface, reaching its highest elevations beneath hills and de-creasing in height toward valleys (Figure 5.26). When you seea wetland (swamp), it indicates that the water table is right atthe surface. Lakes and streams generally occupy areas lowenough that the water table is above the land surface.

Several factors contribute to the irregular surface of thewater table. One important influence is the fact that ground-water moves very slowly. Because of this, water tends to “pileup” beneath high areas between stream valleys. If rainfallwere to cease completely, these water “hills” would slowlysubside and gradually approach the level of the valleys. How-ever, new supplies of rainwater are usually added often

Unsuccessfulwell

Successfulwell

Perchedwater table

Spring

AquitardAquitard

Main water tableMain water table

Aquitard

Main water table

Unsaturatedzone

Zone ofsaturation

FIGURE 5.26 This diagram illustrates the relative positions of many features associated withsubsurface water.

Students Sometimes Ask . . .About how much of a river’s flow is contributed by ground-water?

In one study of 54 streams in all parts of the United States, theanalysis indicated that 52 percent of the streamflow was con-tributed by groundwater. The groundwater contribution rangedfrom a low of 14 percent to a maximum of 90 percent. Ground-water is also a major source of water for lakes and wetlands.

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Distribution and Movement of Groundwater 137

enough to prevent this. Nevertheless, in times of extendeddrought, the water table may drop enough to dry up shallowwells. Other causes for the uneven water table are variationsin rainfall and permeability from place to place.

Factors Influencing the Storage and Movement of GroundwaterThe nature of subsurface materials strongly influences therate of groundwater movement and the amount of ground-water that can be stored. Two factors are especially impor-tant: porosity and permeability.

Porosity Water soaks into the ground because bedrock, sed-iment, and soil contain countless voids or openings. Theseopenings are similar to those of a sponge and are often calledpore spaces. The quantity of groundwater that can be storeddepends on the porosity of the material, which is the per-centage of the total volume of rock or sediment that consistsof pore spaces. Voids most often are spaces between sedi-mentary particles, but also common are joints, faults, cavitiesformed by the dissolving of soluble rock such as limestone,and vesicles (voids left by gases escaping from lava).

Variations in porosity can be great. Sediment is commonlyquite porous, and open spaces may occupy 10 to 50 percentof the sediment’s total volume. Pore space depends on thesize and shape of the grains, how they are packed together,the degree of sorting, and in sedimentary rocks, the amountof cementing material. Where sediments are poorly sorted,the porosity is reduced because finer particles tend to fill theopenings among the larger grains. Most igneous and meta-morphic rocks, as well as some sedimentary rocks, are com-posed of tightly interlocking crystals so the voids betweengrains may be negligible. In these rocks, fractures must pro-vide the voids.

Permeability Porosity alone cannot measure a material’scapacity to yield groundwater. Rock or sediment may be veryporous yet still not allow water to move through it. The poresmust be connected to allow water flow, and they must be largeenough to allow flow. Thus, the permeability of a material,its ability to transmit a fluid, is also very important.

Groundwater moves by twisting and turning throughsmall, interconnected openings. The smaller the pore spaces,the slower the groundwater moves. If the spaces betweenparticles are too small, water cannot move at all. For example,clay’s ability to store water can be great, owing to its highporosity, but its pore spaces are so small that water is unableto move through it. Thus, we say that clay is impermeable.

Aquitards and Aquifers Impermeable layers that hinder orprevent water movement are termed aquitards

Clay is a good example. In contrast,larger particles, such as sand or gravel, have larger porespaces. Therefore, the water moves with relative ease. Per-

water, tard = slow2.1aqua =

meable rock strata or sediments that transmit groundwaterfreely are called aquifers (“water carriers”). Sands and grav-els are common examples. Aquifers are important becausethey are the water-bearing layers sought after by well drillers.

Groundwater MovementThe movement of most groundwater is exceedingly slow,from pore to pore. A typical rate is a few centimeters per day.The energy that makes the water move is provided by theforce of gravity. In response to gravity, water moves fromareas where the water table is high to zones where the watertable is lower (see Box 5.2). This means that water usuallygravitates toward a stream channel, lake, or spring. Althoughsome water takes the most direct path down the slope of thewater table, much of the water follows long, curving paths to-ward the zone of discharge.

Figure 5.27 shows how water percolates into a stream fromall possible directions. Some paths clearly turn upward, ap-parently against the force of gravity, and enter through thebottom of the channel. This is easily explained: The deeperyou go into the zone of saturation, the greater the water pres-sure. Thus, the looping curves followed by water in the sat-urated zone may be thought of as a compromise between thedownward pull of gravity and the tendency of water to movetoward areas of reduced pressure.

Students Sometimes Ask . . .How fast does groundwater move?

The rate of groundwater movement is highly variable. Onemethod of measuring this movement involves introducing dyeinto a well. The time is measured until the coloring agent ap-pears in another well at a known distance from the first. A typi-cal rate is about 15 meters per year (about 4 centimeters per day).

Stream

Water table

FIGURE 5.27 Arrows indicate groundwater movement throughuniformly permeable material. The looping curves may be thought of as a compromise between the downward pull of gravity and the tendency of water to move toward areas of reduced pressure.

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138 C H A P T E R 5 Running Water and Groundwater

SpringsSculpturing Earth’s Surface� Groundwater

Springs have aroused the curiosity and wonder of people forthousands of years. The fact that springs were (and to somepeople still are) rather mysterious phenomena is not difficultto understand, for here is water flowing freely from theground in all kinds of weather in seemingly inexhaustiblesupply but with no obvious source. Today, we know that thesource of springs is water from the zone of saturation andthat the ultimate source of this water is precipitation.

Whenever the water table intersects the ground surface, anatural flow of groundwater results, which we call a spring.Springs such as the one in Figure 5.28 form when an aquitardblocks the downward movement of groundwater and forcesit to move laterally. When the permeable bed (aquifer) out-crops in a valley, a spring or series of springs results.

Another situation that can produce a spring is illustrated inFigure 5.26. Here an aquitard is situated above the main watertable. As water percolates downward, a portion accumulatesabove the aquitard to create a localized zone of saturation anda perched water table. Springs, however, are not confined toplaces where a perched water table creates a flow at the surface.

Many geological situations lead to the formation of springs be-cause subsurface conditions vary greatly from place to place.

Hot SpringsBy definition, the water in hot springs is 6–9°C (10–15°F)warmer than the mean annual air temperature for the local-ities where they occur. In the United States alone, there arewell over 1,000 such springs.

Temperatures in deep mines and oil wells usually rise withan increase in depth averaging about 2°C per 100 meters (1°Fper 100 feet). Therefore, when groundwater circulates at greatdepths, it becomes heated, and if it rises to the surface, thewater may emerge as a hot spring. The water of some hotsprings in the United States, particularly in the east, is heatedin this manner. However, the great majority (over 95 percent)of the hot springs (and geysers) in the United States are foundin the west. The reason for such a distribution is that thesource of heat for most hot springs is cooling igneous rock,and it is in the west that igneous activity has been most recent.

GeysersGeysers are intermittent hot springs or fountains in whichcolumns of water are ejected with great force at various in-tervals, often rising 30 to 60 meters (100 to 200 feet). After the

BOX 5.2 � UNDERSTANDING EARTH

Measuring Groundwater MovementThe foundations of our modern under-standing of groundwater movement beganin the mid-nineteenth century with thework of the French scientist-engineerHenri Darcy. Among the experiments car-ried out by Darcy was one that showedthat the velocity of groundwater flow is

proportional to the slope of the watertable—the steeper the slope, the faster thewater moves (because the steeper theslope, the greater the pressure differencebetween two points). The water-tableslope is known as the hydraulic gradientand can be expressed as follows:

Where is the elevation of one point onthe water table, is the elevation of a sec-ond point, and d is the horizontal distancebetween the two points (Figure 5.B).

Darcy also discovered that the flow ve-locity varied with the permeability of thesediment—groundwater flows more rap-idly through sediments having greater per-meability than through materials havinglower permeability. This factor is known ashydraulic conductivityand is a coefficient that

takes into account the permeability of theaquifer and the viscosity of the fluid.

To determine discharge (Q)—that is, theactual volume of water that flows throughan aquifer in a specified time—the follow-ing equation is used:

Where is the hydraulic gradient,

K is the coefficient that represents hydraulicconductivity, and A is the cross-sectionalarea of the aquifer. This expression has cometo be called Darcy’s law .

h1 - h2

d

Q =

K A1 h1 - h22

d.

h2

h1

hydraulic gradient =

h1 - h2

d.

h1 – h2

d

h1 – h2

h2

h1

dh2

h1

d

Wells

Water table

Wells

Hydraulic gradient =

FIGURE 5.B The hydraulic gradient is determined by measuring the difference inelevation between two points on the water table divided by the distancebetween them, d. Wells are used to determine the height of the water table.

1h1 - h22

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Wells 139

jet of water ceases, a column of steam rushes out, usually witha thundering roar. Figure 5.29 shows a wintertime eruptionof Old Faithful in Yellowstone National Park, perhaps themost famous geyser in the world, which erupts about onceeach hour. Geysers are also found in other parts of the world,including New Zealand and Iceland. In fact, the Icelandicword geysa—to gush—gives us the name geyser.

Geysers occur where extensive underground chambersexist within hot igneous rocks. How they operate is shown inFigure 5.30. As relatively cool groundwater enters the cham-bers, it is heated by the surrounding rock. At the bottom of thechamber, the water is under great pressure because of theweight of the overlying water. This great pressure preventsthe water from boiling at the normal surface temperature of100°C (212°F). For example, at the bottom of a 300-meter(1,000-foot) water-filled chamber, water must attain a tem-perature of nearly 230°C (450°F) before it will boil. The heat-ing causes the water to expand, with the result that some isforced out at the surface. This loss of water reduces the pres-sure on the remaining water in the chamber, which lowersthe boiling point. A portion of the water deep within thechamber quickly turns to steam and causes the geyser toerupt. Following the eruption, cool groundwater again seepsinto the chamber, and the cycle begins anew.

WellsSculpturing Earth’s Surface� Groundwater

The most common method for removing groundwater isthe well, a hole bored into the zone of saturation (see Figure5.26). Wells serve as small reservoirs into which ground-water moves and from which it can be pumped to the sur-face. The use of wells dates back many centuries andcontinues to be an important method of obtaining watertoday. By far the single greatest use of this water in theUnited States is irrigation for agriculture. More than 65 per-cent of the groundwater used each year is for this purpose.Industrial uses rank a distant second, followed by theamount used by homes in cities and rural areas.

The level of the water table may fluctuate considerablyduring the course of a year, dropping during dry seasons andrising following periods of rain. Therefore, to ensure a con-tinuous supply of water, a well must penetrate far below thewater table. Whenever a substantial amount of water is with-drawn from a well, the water table around the well is low-ered. This effect, termed drawdown, decreases withincreasing distance from the well. The result is a depressionin the water table, roughly conical in shape, known as a coneof depression (Figure 5.31). For most small domestic wells,the cone of depression is negligible. However, when wellsare used for irrigation or for industrial purposes, the with-drawal of water can be great enough to create a very wideand steep cone of depression that may substantially lower

FIGURE 5.28 Spring flowing from a valley wall in Arizona’s MarbleCanyon. (Photo by Michael Collier)

FIGURE 5.29 A wintertime eruption of Old Faithful, one of the world’smost famous geysers. It emits as much as 45,000 liters (almost 12,000gallons) of hot water and steam about once each hour. (Photo by MarcMuench/David Muench Photography, Inc.)

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Dry wellDry well

Water table

Site of newhigh-capacity

well

Lowered water table

WellWell

A. Before heavypumping

B. After heavypumping

Cone of d

epression

FIGURE 5.31 A cone of depressionin the water table often forms around apumping well. If heavy pumping lowersthe water table, some wells may be leftdry.

Water tableCavern

Warm ashand

lava flowsHeat flow

A.

B.

C.

Outflow

Steam

Steam

Heat flow

Geysereruption

Emptychambers

Heat flow

Steam

TIM

E

FIGURE 5.30 Idealized diagramsillustrating the stages in the eruption cycleof a geyser. A. Groundwater entersunderground caverns and fractures in hotigneous rock, where it is heated to near itsboiling point. B. Heating causes the water toexpand, with some being forced out at thesurface. The loss of water reduces thepressure on the remaining water, thusreducing its boiling temperature. Some ofthe water flashes to steam. C. The rapidlyexpanding steam forces the hot water out ofthe chambers to produce a geyser. Theempty chambers fill again, and the cyclestarts anew.

140 C H A P T E R 5 Running Water and Groundwater

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Artesian Wells 141

the water table in an area and cause nearby shallow wells tobecome dry. Figure 5.31 illustrates this situation.

dry periods. However, in some wells, water rises, sometimesoverflowing at the surface.

The term artesian is applied to any situation in whichgroundwater rises in a well above the level where it was ini-tially encountered. For such a situation to occur, two condi-tions usually exist (Figure 5.32): (1) Water is confined to anaquifer that is inclined so that one end is exposed at the sur-face, where it can receive water; and (2) aquitards both aboveand below the aquifer must be present to prevent the waterfrom escaping. Such an aquifer is called a confined aquifer.When such a layer is tapped, the pressure created by theweight of the water above will force the water to rise. If therewere no friction, the water in the well would rise to the levelof the water at the top of the aquifer. However, friction reducesthe height of this pressure surface. The greater the distancefrom the recharge area (area where water enters the inclinedaquifer), the greater the friction and the less the rise of water.

In Figure 5.32, Well 1 is a nonflowing artesian well, because atthis location the pressure surface is below ground level. Whenthe pressure surface is above the ground and a well is drilledinto the aquifer, a flowing artesian well is created (Well 2, Figure5.32). Not all artesian systems are wells. Artesian springs alsoexist. In such situations groundwater may reach the surface byrising along a natural fracture such as a fault rather than throughan artificially produced hole. In deserts, artesian springs aresometimes responsible for creating an oasis.

Artesian systems act as conduits, transmitting water fromremote areas of recharge great distances to the points of dis-charge. In this manner, water that fell in central Wisconsin yearsago is now taken from the ground and used by communities

many kilometers to the south inIllinois. In South Dakota, sucha system brings water eastwardacross the state from the BlackHills in the west.

On a different scale, citywater systems may be consid-ered examples of artificial arte-sian systems (Figure 5.33). Thewater tower, into which wateris pumped, may be consideredthe area of recharge, the pipesthe confined aquifer, and thefaucets in homes the flowingartesian wells.

EnvironmentalProblemsAssociated withGroundwaterAs with many of our valuablenatural resources, groundwa-ter is being exploited at an in-creasing rate. In some areas,overuse threatens the ground-

Nonflowing artesian well(water must be pumped from pressure surface to surface)

Recha

rge a

rea

Recha

rge a

rea

#1#1

AquiferAquiferAquitardAquitard

#2#2

Recha

rge a

rea

#1Pressure surface

AquitardAquitardAquitardAquiferAquitard

#2

Nonflowingartesian well

Flowingartesian well

Pressuresurface

Recharge area

Pressuresurface

Flowingartesian well

FIGURE 5.32 Artesian systems occur when an inclined aquifer is surrounded by impermeable beds.

Students Sometimes Ask . . .I have heard people say that supplies of groundwater can belocated using a forked stick. Can this actually be done?

What you describe is a practice called “water dowsing.” In theclassic method, a person holding a forked stick walks back andforth over an area. When water is detected, the bottom of the “Y”is supposed to be attracted downward.

Geologists and engineers are dubious, to say the least. Casehistories and demonstrations may seem convincing, but whendowsing is exposed to scientific scrutiny, it fails. Most “success-ful” examples of water dowsing occur in places where waterwould be hard to miss. In a region of adequate rainfall and fa-vorable geology, it is difficult to drill and not find water!

Artesian WellsSculpturing Earth’s Surface� Groundwater

In most wells, water does not rise on its own. If water is firstencountered at 30 meters of depth, it remains at that level,fluctuating perhaps a meter or two with seasonal wet and

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142 C H A P T E R 5 Running Water and Groundwater

water supply. In other places, groundwater withdrawal hascaused the ground and everything resting upon it to sink. Stillother localities are concerned with the possible contaminationof their groundwater supply.

Treating Groundwater as a Nonrenewable ResourceFor many, groundwater appears to be an endlessly renew-able resource, for it is continually replenished by rainfall andmelting snow. But in some regions, groundwater has been

and continues to be treated as a nonrenewable resource. Wherethis occurs, the amount of water available to recharge theaquifer is significantly less than the amount being withdrawn.

The High Plains, a relatively dry region that extends fromSouth Dakota to western Texas, provides one example (Figure5.34). Here an extensive agricultural economy is largely de-pendent on irrigation. In some parts of the region where in-tense irrigation has been practiced for an extended period,depletion of groundwater has been severe. Under these cir-cumstances, it can be said that the groundwater is literallybeing “mined.” Even if pumping were to cease immediately,it would take thousands of years for the groundwater to befully replenished. Groundwater depletion has been a con-cern in the High Plains and other areas of the West for manyyears, but the problem is not confined to this part of the coun-try. Increasing demands on groundwater resources haveoverstressed aquifers in many areas, not just in arid and semi-arid regions.

Land Subsidence Caused by Groundwater WithdrawalAs you will see later in this chapter, surface subsidence canresult from natural processes related to groundwater. How-ever, the ground may also sink when water is pumped fromwells faster than natural recharge processes can replace it.This effect is particularly pronounced in areas underlain bythick layers of loose sediments. As water is withdrawn, theground subsides because the weight of the overburden packs

the sediment grains moretightly together.

Many areas can be used to illustrate land subsidencecaused by excessive pumpingof groundwater from rela-tively loose sediment. A classicexample in the United Statesoccurred in the San JoaquinValley of California (Figure5.35). This important agricul-tural region relies heavily on ir-rigation. Land subsidence dueto groundwater withdrawalbegan in the valley in the mid-1920s and locally exceeded8 meters (28 feet) by 1970. Then,because of the importation ofsurface water and a decrease ingroundwater pumping, waterlevels in the aquifer recoveredand subsidence ceased. How-ever, during a drought in 1976and 1977, heavy groundwaterpumping led to renewed sub-sidence. This time, water levelsdropped at a much faster ratethan during the previous

MN

IA

SDWY

CONM

TX

OK

KS

NE

Mexico

R O

C K

Y

M O

U N

T A

I N

S

O g

a l

l a l

a

F o

r m

a t

i o

n

A.

B.C.

FIGURE 5.34 A. The High Plains extend from the western Dakotas south to Texas. Despite being a land of little rain, this is an important agricultural region. The reason is a vast endowment of ground-water that makes irrigation possible through most of the region. The source of most of this water is the Ogallala formation, the largest aquifer in the United States. B. In some agricultural regions, water is pumped from the ground faster than it is replenished. In such instances, groundwater is being treated as a nonrenewable resource. This aerial view shows circular crop fields irrigated by center pivot irrigation systems in semiarid eastern Colorado. (Photo by James L. Amos/Corbis/Bettmann)C. Groundwater provides more than 190 billion liters (50 billion gallons) per day in support of the agricultural economy of the United States. (Photo by Michael Collier)

Water ispumped into

tank

Pressure surface(level to which water

will rise)

Pressure moveswater through pipe

Water tank

Well

FIGURE 5.33 City water systems can be considered to be artificialartesian systems.

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Environmental Problems Associated with Groundwater 143

period, because of the reducedstorage capacity caused by ear-lier compaction of material inthe aquifer. In all, more than13,400 square kilometers (5,200square miles) of irrigableland—one half of the entire val-ley—were affected by subsi-dence. Damage to structures,including highways, bridges,water lines, and wells, was ex-tensive. Many other examplesof land subsidence due togroundwater pumping occurin the United States and else-where in the world.

GroundwaterContaminationThe pollution of groundwateris a serious matter, particularlyin areas where aquifers pro-vide a large part of the watersupply. One common sourceof groundwater pollution is

sewage. Its sources include an ever increasing number of sep-tic tanks, as well as farm wastes and inadequate or brokensewer systems.

If sewage water that is contaminated with bacteria entersthe groundwater system, it may become purified throughnatural processes. The harmful bacteria can be mechanicallyfiltered by the sediment through which the water percolates,destroyed by chemical oxidation, and/or assimilated by otherorganisms. For purification to occur, however, the aquifermust be of the correct composition.

For example, extremely permeable aquifers (such as highlyfractured crystalline rock, coarse gravel, or cavernous lime-stone) have such large openings that contaminated ground-water may travel long distances without being cleansed. Inthis case, the water flows too rapidly and is not in contactwith the surrounding material long enough for purificationto occur. This is the problem at Well 1 in Figure 5.36A.

San Joaquin Valley

CA

FIGURE 5.35 The shaded area on the map shows California’sSan Joaquin Valley. The marks on the utility pole in the photoindicate the level of the surrounding land in preceding years.Between 1925 and 1975 this part of the San Joaquin Valleysubsided almost 9 meters because of the withdrawal of ground-water and the resulting compaction of sediments. (Photo courtesyof U.S. Geological Survey)

Students Sometimes Ask . . .Does land subsidence caused by groundwater withdrawalaffect a very large area?

According to an estimate by the U.S. Geological Survey, it is sub-stantial. In the contiguous 48 states, the area amounts to approx-imately 26,000 square kilometers (more than 10,000 squaremiles)—an area about the same size as the state of Massachusetts!

Well 1 deliveringcontaminated water

Septictank

A.

B.

Well 2 deliveringclean water

Contaminatedwater

Contaminatedwater

Permeablesandstone

Watertable

Watertable

Cavernouslimestone

Septictank

FIGURE 5.36 A. Although the contaminated water has traveled more than 100 meters beforereaching Well 1, the water moves too rapidly through the cavernous limestone to be purified. B. Asthe discharge from the septic tank percolates through the permeable sandstone, it is purified in arelatively short distance.

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Conversely, when the aquifer is composed of sand or per-meable sandstone, the water can sometimes be purified aftertraveling only a few dozen meters through it. The openingsbetween sand grains are large enough to permit water move-ment, yet the movement of the water is slow enough to allowample time for its purification (Well 2, Figure 5.36B).

Other sources and types of contamination also threatengroundwater supplies (Figure 5.37). These include widelyused substances such as highway salt, fertilizers that arespread across the land surface, and pesticides. In addition, awide array of chemicals and industrial materials may leakfrom pipelines, storage tanks, landfills, and holding ponds.Some of these pollutants are classified as hazardous, meaningthat they are either flammable, corrosive, explosive, or toxic.As rainwater oozes through the refuse, it may dissolve a va-riety of potential contaminants. If the leached material reachesthe water table, it will mix with the groundwater and con-taminate the supply.

Because groundwater movement is usually slow, pollutedwater might go undetected for a long time. In fact, contami-nation is sometimes discovered only after drinking water hasbeen affected and people become ill. By this time, the volumeof polluted water might be very large, and even if the sourceof contamination is removed immediately, the problem is notsolved. Although the sources of groundwater contaminationare numerous, there are relatively few solutions.

Once the source of the problem has been identified andeliminated, the most common practice is simply to abandonthe water supply and allow the pollutants to be flushed awaygradually. This is the least costly and easiest solution, but theaquifer must remain unused for many years. To accelerate thisprocess, polluted water is sometimes pumped out and treated.

Following removal of the taintedwater, the aquifer is allowed torecharge naturally or, in somecases, the treated water or otherfreshwater is pumped back in.This process is costly, time-consuming, and it may be risky be-cause there is no way to be certainthat all of the contamination hasbeen removed. Clearly, the mosteffective solution to groundwatercontamination is prevention.

The Geologic Workof GroundwaterGroundwater dissolves rock. Thisfact is key to understanding howcaverns and sinkholes form. Be-cause soluble rocks, especiallylimestone, underlie millions ofsquare kilometers of Earth’s sur-face, it is here that groundwatercarries on its important role as an

erosional agent. Limestone is nearly insoluble in pure waterbut is quite easily dissolved by water containing small quan-tities of carbonic acid. Most natural water contains this weakacid because rainwater readily dissolves carbon dioxide fromthe air and from decaying plants. Therefore, when ground-water comes in contact with limestone, the carbonic acid re-acts with calcite in the rocks to form calcium bicarbonate, asoluble material that is then carried away in solution.

CavernsThe most spectacular results of groundwater’s erosionalhandiwork are limestone caverns. In the United States aloneabout 17,000 caves have been discovered. Although most arerelatively small, some have spectacular dimensions. Carls-bad Caverns in southeastern New Mexico and MammothCave in Kentucky are famous examples. One chamber inCarlsbad Caverns has an area equivalent to 14 football fieldsand enough height to accommodate the U.S. Capitol Building.At Mammoth Cave, the total length of interconnected cav-erns extends for more than 540 kilometers (340 miles).

Most caverns are created at or below the water table in thezone of saturation. Here acidic groundwater follows lines ofweakness in the rock, such as joints and bedding planes. Astime passes, the dissolving process slowly creates cavitiesand gradually enlarges them into caverns. Material that isdissolved by the groundwater is eventually discharged intostreams and carried to the ocean.

Certainly the features that arouse the greatest curiosity formost cavern visitors are the stone formations that give somecaverns a wonderland appearance. These are not ero-sional features, like the caverns in which they reside, but

A.

B.

FIGURE 5.37 Sometimes agricultural chemicals A. and materials leached from landfills B. find their way into the groundwater. These are two potential sources of groundwater contamination. (Photo A by Roy Morsch/The Stock Market; Photo B by F. Rossotto/The Stock Market)

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The Geologic Work of Groundwater 145

depositional features. They are created by the seemingly end-less dripping of water over great spans of time. The calciumcarbonate that is left behind produces the limestone we calltravertine. These cave deposits, however, are also commonlycalled dripstone, an obvious reference to their mode of origin.

Although the formation of caverns takes place in the zoneof saturation, the deposition of dripstone is not possible untilthe caverns are above the water table in the unsaturated zone.This commonly occurs as nearby streams cut their valleysdeeper, lowering the water table as the elevation of the riversdrops. As soon as the chamber is filled with air, the conditionsare right for the decoration phase of cavern building to begin.

Of the various dripstone features found in caverns, per-haps the most familiar are stalactites. These icicle-like pen-dants hang from the ceiling of the cavern and form wherewater seeps through cracks above. When water reaches airin the cave, some of the dissolved carbon dioxide escapesfrom the drop and calcite begins to precipitate. Depositionoccurs as a ring around the edge of the water drop. As dropafter drop follows, each leaves an infinitesimal trace of calcitebehind, and a hollow limestone tube is created. Water thenmoves through the tube, remains suspended momentarily atthe end, contributes a tiny ring of calcite, and falls to the cav-

ern floor. The stalactite just described is appropriately calleda soda straw (Figure 5.38A). Often the hollow tube of the sodastraw becomes plugged or its supply of water increases. In ei-ther case, the water is forced to flow and deposit along theoutside of the tube. As deposition continues, the stalactitetakes on the more common conical shape.

Formations that develop on the floor of a cavern and reachupward toward the ceiling are called stalagmites (Figure5.38B). The water supplying the calcite for stalagmite growthfalls from the ceiling and splatters over the surface. As a re-sult, stalagmites do not have a central tube and are usuallymore massive in appearance and more rounded on theirupper ends than stalactites. Given enough time, a downward-growing stalactite and an upward-growing stalagmite mayjoin to form a column.

Karst TopographyMany areas of the world have landscapes that to a large ex-tent have been shaped by the dissolving power of ground-water. Such areas are said to exhibit karst topography, namedfor the Krs region in Slovenia where such topography is strik-ingly developed. In the United States, karst landscapes occur

A.

B.

FIGURE 5.38 A. “Live” solitary soda-straw stalactites. Lehman Caves. Great Basin National Park,Nevada. (Photo by Tom Bean) B. Stalagmites grow upward from the cavern floor. Chinese Theater,Carlsbad Caverns National Park, New Mexico. (Photo by David Muench Photography, Inc.)

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146 C H A P T E R 5 Running Water and Groundwater

in many areas that are underlain by limestone, including por-tions of Kentucky, Tennessee, Alabama, southern Indiana,and central and northern Florida (Figure 5.39). Generally, aridand semiarid areas do not develop karst topography becausethere is insufficient groundwater. When solution features existin such regions, they are likely to be remnants of a time whenrainier conditions prevailed.

Karst areas typically have irregular terrain punctuatedwith many depressions called sinkholes or, simply, sinks (seeFigure 5.25B, p. 135). In the limestone areas of Florida, Ken-tucky, and southern Indiana, there are literally tens of thou-

sands of these depressionsvarying in depth from just ameter or two to a maximum ofmore than 50 meters.

Sinkholes commonly formin one of two ways. Some de-velop gradually over manyyears without any physicaldisturbance to the rock. Inthese situations, the limestoneimmediately below the soil isdissolved by downward-seep-ing rainwater that is freshlycharged with carbon dioxide.These depressions are usuallynot deep and are characterizedby relatively gentle slopes. Bycontrast, sinkholes can alsoform suddenly and withoutwarning when the roof of acavern collapses under its ownweight. Typically, the depres-sions created in this mannerare steep-sided and deep.When they form in populousareas, they may represent a se-rious geologic hazard. Such asituation is clearly the case inFigure 5.40.

In addition to a surface pock-marked by sinkholes, karst re-gions characteristically show astriking lack of surface drainage(streams). Following a rainfall,runoff is quickly funneled be-low ground through sinks. Itthen flows through cavernsuntil it finally reaches the watertable. Where streams do exist atthe surface, their paths are usu-ally short. The names of suchstreams often give a clue totheir fate. In the MammothCave area of Kentucky, for ex-ample, there is Sinking Creek,Little Sinking Creek, and Sink-

ing Branch. Some sinkholes become plugged with clay and de-bris, creating small lakes or ponds.

Some regions of karst development exhibit landscapesthat look very different from the sinkhole-studded terraindepicted in Figure 5.39. One striking example is an extensiveregion in southern China that is described as exhibitingtower karst. As Figure 5.41 shows, the term tower is appro-priate because the landscape consists of a maze of isolatedsteep-sided hills that rise abruptly from the ground. Eachis riddled with interconnected caves and passageways. Thistype of karst topography forms in wet tropical and

Watertable

Sinkingstream

Watertable

A.

B.

Sinkholes

Sinkingstream

Collapsesink

C.

Sink holes

Sink holes

Watertable

Sinkingstream

Springs

Solutionvalley

Limestone

TIM

E

FIGURE 5.39 Development of a karst landscape. A. During early stages, groundwater percolates through limestone along joints and bedding planes. Solution activity creates and enlarges caverns at and below the water table. B. In this view, sinkholes are well developed and surface streams are funneled below ground. C. With the passage of time, caverns grow larger and the number and size of sinkholes increase. Collapse of caverns and coalescence of sinkholes form larger, flat-floored depressions. Eventually, solution activity may remove most of the limestone from the area, leaving only isolated remnants as in Figure 5.41.

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subtropical regions having thick beds of highly jointed lime-stone. Here groundwater has dissolved large volumes oflimestone, leaving only these residual towers. Karst devel-opment is more rapid in tropical climates due to the abun-dant rainfall and the greater availability of carbon dioxidefrom the decay of lush tropical vegetation. The extra carbondioxide in the soil means there is more carbonic acid for dis-solving limestone. Other tropical areas of advanced karstdevelopment include portions of Puerto Rico, western Cuba,and northern Vietnam.

FIGURE 5.40 This small sinkhole formed suddenly in 1991 when theroof of a cavern collapsed, destroying this home in Frostproof, Florida.(Photo by St. Petersburg Times/Liaison Agency, Inc.)

FIGURE 5.41 One of the best-known and most distinctive regions oftower karst development is the Guilin District of southeastern China.(Photo by A. C. Waltham/Robert Harding World Imagery)

Students Sometimes Ask . . .Is limestone the only rock type that develops karst features?

No. For example, karst development can occur in other carbonaterocks such as marble and dolostone. In addition, evaporates suchas gypsum and salt (halite) are highly soluble and are readily dis-solved to form karst features, including sinkholes, caves, and dis-appearing streams. This latter situation is termed evaporite karst.

Chapter Summary

� The hydrologic cycle describes the continuous interchange ofwater among the oceans, atmosphere, and continents. Pow-ered by energy from the Sun, it is a global system in whichthe atmosphere provides the link between the oceans andcontinents. The processes involved in the hydrologic cycleinclude precipitation, evaporation, infiltration (the movementof water into rocks or soil through cracks and pore spaces),runoff (water that flows over the land rather than infiltrat-ing into the ground), and transpiration (the release of watervapor to the atmosphere by plants). Running water is the sin-gle most important agent sculpturing Earth’s land surface.

� The land area that contributes water to a stream is itsdrainage basin. Drainage basins are separated by imagi-nary lines called divides.

� River systems consist of three main parts: the zones oferosion, transportation, and deposition.

� The factors that determine a stream’s velocity are gradient(slope of the stream channel), shape, size, and roughness ofthe channel, and the stream’s discharge (amount of waterpassing a given point per unit of time, frequently meas-ured in cubic feet per second). Most often, the gradientand roughness of a stream decrease downstream, whilewidth, depth, discharge, and velocity increase.

� Streams transport their load of sediment in solution(dissolved load), in suspension (suspended load), and alongthe bottom of the channel (bed load). Much of the dissolved

load is contributed by groundwater. Most streams carrythe greatest part of their load in suspension. The bed loadmoves only intermittently and is usually the smallest por-tion of a stream’s load.

� A stream’s ability to transport solid particles is describedusing two criteria: capacity (the maximum load of solidparticles a stream can carry) and competence (the maxi-mum particle size a stream can transport). Competenceincreases as the square of stream velocity, so if velocitydoubles, water’s force increases fourfold.

� Streams deposit sediment when velocity slows and compe-tence is reduced. This results in sorting, the process by whichlike-sized particles are deposited together. Stream depositsare called alluvium and may occur as channel deposits calledbars, as floodplain deposits, which include natural levees, andas deltas or alluvial fans at the mouths of streams.

� Stream channels are of two basic types: bedrock channelsand alluvial channels. Bedrock channels are most commonin headwaters regions where gradients are steep. Rapidsand waterfalls are common features. Two types of allu-vial channels are meandering channels and braided channels.

� The two general types of base level (the lowest point towhich a stream may erode its channel) are (1) ultimate baselevel and (2) temporary, or local, base level. Any change inbase level will cause a stream to adjust and establish anew balance. Lowering base level will cause a stream to

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148 C H A P T E R 5 Running Water and Groundwater

downcut, whereas raising base level results in depositionof material in the channel.

� When a stream has cut its channel closer to base level, its en-ergy is directed from side to side, and erosion produces aflat valley floor, or floodplain. Streams that flow upon flood-plains often move in sweeping bends called meanders.Widespread meandering may result in shorter channel seg-ments, called cutoffs, and/or abandoned bends, calledoxbow lakes.

� Floods are triggered by heavy rains and/or snowmelt.Sometimes human interference can worsen or even causefloods. Flood-control measures include the building ofartificial levees and dams, as well as channelization, whichcould involve creating artificial cutoffs. Many scientists andengineers advocate a nonstructural approach to flood con-trol that involves more appropriate land use.

� Common drainage patterns produced by streams include(1) dendritic, (2) radial, (3) rectangular, and (4) trellis.

� As a resource, groundwater represents the largest reservoir offreshwater that is readily available to humans. Geologically,the dissolving action of groundwater produces caves andsinkholes. Groundwater is also an equalizer of streamflow.

� Groundwater is water that occupies the pore spaces insediment and rock in a zone beneath the surface calledthe zone of saturation. The upper limit of this zone is thewater table. The unsaturated zone is above the water tablewhere the soil, sediment, and rock are not saturated.

� The quantity of water that can be stored depends on theporosity (the volume of open spaces) of the material. Thepermeability (the ability to transmit a fluid through inter-

connected pore spaces) of a material is a very importantfactor controlling the movement of groundwater.

� Materials with very small pore spaces (such as clay) hin-der or prevent groundwater movement and are calledaquitards. Aquifiers consist of materials with larger porespaces (such as sand) that are permeable and transmitgroundwater freely.

� Springs occur whenever the water table intersects the landsurface and a natural flow of groundwater results. Wells,openings drilled into the zone of saturation, withdrawgroundwater and create roughly conical depressions inthe water table known as cones of depression. Artesian wellsoccur when water rises above the level at which it wasinitially encountered.

� When groundwater circulates at great depths, it becomesheated. If it rises, the water may emerge as a hot spring.Geysers occur when groundwater is heated in undergroundchambers, expands, and some water quickly changes tosteam, causing the geyser to erupt. The source of heat formost hot springs and geysers is hot igneous rock.

� Some of the current environmental problems involvinggroundwater include (1) overuse by intense irrigation, (2) land subsidence caused by groundwater withdrawal,and (3) contamination by pollutants.

� Most caverns form in limestone at or below the water tablewhen acidic groundwater dissolves rock along lines ofweakness, such as joints and bedding planes. Karst topog-raphy exhibits an irregular terrain punctuated with manydepressions, called sinkholes.

Key Termsalluvial fan (p. 132)alluvium (p. 124)aquifer (p. 137)aquitard (p. 137)artesian well (p. 141)backswamp (p. 129)bar (p. 129)base level (p. 126)bed load (p. 122)braided stream (p. 125)capacity (p. 123)cavern (p. 144)competence (p. 123)cone of depression (p. 139)cut bank (p. 125)cutoff (p. 125)

delta (p. 129)dendritic pattern (p. 132)discharge (p. 120)dissolved load (p. 122)distributary (p. 129)divide (p. 118)drainage basin (p. 118)drawdown (p. 139)evapotranspiration (p. 118)flood (p. 132)floodplain (p. 128)geyser (p. 138)gradient (p. 120)groundwater (p. 136)hot spring (p. 138)hydrologic cycle (p. 117)

incised meander (p. 128)infiltration (p. 117)infiltration capacity (p. 118)karst topography (p. 145)laminar flow (p. 119)meander (p. 125)natural levee (p. 129)oxbow lake (p. 125)permeability (p. 137)point bar (p. 125)porosity (p. 137)radial pattern (p. 132)rectangular pattern (p. 132)runoff (p. 117)saltation (p. 123)settling velocity (p. 123)

sinkhole (sink) (p. 146)sorting (p. 124)spring (p. 138)stalactite (p. 145)stalagmite (p. 145)stream valley (p. 127)suspended load (p. 122)transpiration (p. 117)trellis pattern (p. 132)turbulent flow (p. 119)unsaturated zone (p. 136)water table (p. 136)well (p. 139)yazoo tributary (p. 129)zone of saturation (p. 136)

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Review Questions 149

Review Questions1. Describe the movement of water through the hydrologic

cycle. Once precipitation has fallen on land, what pathsare available to it?

2. What are the three main parts (zones) of a river system?3. A stream starts out 2,000 meters above sea level and trav-

els 250 kilometers to the ocean. What is its average gra-dient in meters per kilometer?

4. Suppose that the stream mentioned in Question 3 devel-oped extensive meanders so that its course was length-ened to 500 kilometers. Calculate its new gradient. Howdoes meandering affect gradient?

5. When the discharge of a stream increases, what happensto the stream’s velocity?

6. In what three ways does a stream transport its load?7. If you collect a jar of water from a stream, what part of its

load will settle to the bottom of the jar? What portion willremain in the water? What part of a stream’s load wouldprobably not be present in your sample?

8. Differentiate between competency and capacity.9. Are bedrock channels more likely to be found near the

head or near the mouth of a stream?10. Describe a situation that might cause a stream channel

to become braided.11. Define base level. Name the main river in your area. For

what streams does it act as base level? What is the baselevel for the Mississippi River? The Missouri River?

12. Describe two situations that would trigger the formationof incised meanders.

13. Briefly describe the formation of a natural levee. How isthis feature related to backswamps and yazoo tributaries?

14. List two major depositional features, other than naturallevees, that are associated with streams. Under what cir-cumstances does each form?

15. List and briefly describe three basic flood-control strate-gies. What are some drawbacks of each?

16. Each of the following statements refers to a particulardrainage pattern. Identify the pattern. (a) Streams di-verge from a central high area such as a volcano. (b) Streams form a branching, treelike pattern. (c) A pat-tern that develops when bedrock is crisscrossed byjoints and faults.

17. What percentage of freshwater is groundwater (see Table5.2)? If glacial ice is excluded and only liquid freshwateris considered, about what percentage is groundwater?

18. Geologically, groundwater is important as an erosionalagent. Name another significant geological role ofgroundwater.

19. Define groundwater and relate it to the water table.20. How do porosity and permeability differ?21. Distinguish between an aquifer and an aquitard.22. What is the source of heat for most hot springs and geysers?

How is this reflected in the distribution of these features?23. What is meant by the term artesian? In order for artesian

wells to exist, two conditions must be present. List theseconditions.

24. What problem is associated with the pumping ofgroundwater for irrigation in the southern part of theHigh Plains?

25. Briefly explain what happened in the San Joaquin Val-ley of California as the result of excessive groundwaterwithdrawal.

26. Which would be most effective in purifying pollutedgroundwater: an aquifer composed mainly of coarsegravel, sand, or cavernous limestone?

27. Differentiate between stalactites and stalagmites. Howdo these features form?

28. If you were to explore an area that exhibited karst to-pography, what features might you find? This area wouldprobably be underlain by what rock type? Name a regionthat exhibits such features.

29. Describe two ways in which sinkholes are created.

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150 C H A P T E R 5 Running Water and Groundwater

Examining the Earth System1. List the process(es) involved in moving water through

the hydrologic cycle from the (a) hydrosphere to the at-mosphere, (b) atmosphere to the geosphere, (c) biosphereto the atmosphere, and (d) hydrosphere (land) to thehydrosphere (ocean).

2. Over the oceans, evaporation exceeds precipitation, yetsea level does not drop. Why?

3. List at least three specific examples of interactions be-tween humans and the hydrologic cycle (e.g., the con-struction of a dam). Briefly describe the consequence(s)of each of these interactions.

4. Describe the role of the atmosphere, geosphere, and bio-sphere in determining the quantity and quality ofgroundwater available for human consumption.

Online Study GuideThe Earth Science Website uses the resources and flexibilityof the Internet to aid in your study of the topics in this chap-ter. Written and developed by Earth science instructors, thissite will help improve your understanding of Earth science.Visit http://www.prenhall.com/tarbuck and click on the coverof Earth Science 12e to find:

• Online review quizzes• Critical thinking exercises• Links to chapter-specific Web resources• Internet-wide key term searches

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GEODe: Earth Science 151

GEODe: Earth ScienceGEODe: Earth Science makes studying more effective by re-inforcing key concepts using animation, video, narration, in-

teractive exercises, and practice quizzes. A copy is includedwith every copy of Earth Science 12e.

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