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ErosionErosion
►The removal of rock material The removal of rock material (weathered rock, boulders, gravel, (weathered rock, boulders, gravel, sand, silt and clay)sand, silt and clay)
►Erosion suggests transport, but Erosion suggests transport, but transport is not necessarily erosion.transport is not necessarily erosion.
TransportTransport
►Weathered rock and sediments and Weathered rock and sediments and dissolved minerals are moved from the dissolved minerals are moved from the source area to the depositional area.source area to the depositional area.
►Transport agentsTransport agents Running WaterRunning Water WindWind IceIce
DepositionDeposition
►Deposition occurs when the energy of Deposition occurs when the energy of the sediment transport agent the sediment transport agent decreases.decreases.
►Deposition occurs when the sediment Deposition occurs when the sediment can no longer be moved.can no longer be moved.
LithificationLithification
• Lithification involves compaction and Lithification involves compaction and cementation that convert sediment cementation that convert sediment into sedimentary rock. into sedimentary rock.
• Silica and calcium carbonate are the Silica and calcium carbonate are the most common chemical cements, but most common chemical cements, but iron oxide and iron hydroxide cements iron oxide and iron hydroxide cements are important in some rocks.are important in some rocks.
RiversRivers
►Rills to Rivers (channel flow)Rills to Rivers (channel flow)►Sources: Sources:
sheet flow from rain and snowsheet flow from rain and snow groundwatergroundwater soil moisturesoil moisture
►Topographic High to Low (eventually to Topographic High to Low (eventually to sea level—base level)sea level—base level)
►Drainage Basin (all sheet flow and Drainage Basin (all sheet flow and channel flow ends up in one major channel flow ends up in one major channel flow)channel flow)
Fig. 12-3, p. 274
Water Cycle
RiversRivers
►Rivers erode vertically and laterallyRivers erode vertically and laterally►Rivers transport sediments and Rivers transport sediments and
dissolved mineralsdissolved minerals►Rivers deposit sediments Rivers deposit sediments
Figure 1a, p. 273
Hoover Dam and Lake Mead
Erosion or deposition?
Fig. 12-1, p. 270
Note v-shaped valley shape
Note steep gradient (much downcutting, cannot meander)
Erosion or deposition?
Fig. 12-CO, p. 268
What happens when Niagara slips?
Niagara Falls!
http://www.niagaraparks.com/nfgg/geology.php
http://www.lockport-ny.com/Museum/rocks.htm
Horseshoe Falls
How do rivers erode?How do rivers erode?
►Cutting power of rivers is recognized Cutting power of rivers is recognized over time and during periods of over time and during periods of excessive flow (e.g., flood stage)excessive flow (e.g., flood stage)
►Steeper gradients increase Steeper gradients increase downcuttingdowncutting
►Shallow gradients decrease Shallow gradients decrease downcutting but increase lateral flow.downcutting but increase lateral flow.
Fig. 12-4, p. 274
Rapid stream velocity, bed load is coarse, erosion is downward.
Slow stream velocity, bed load is fine, erosion is lateral, stream meanders
Stream Gradient
head
mouth
Fig. 12-5b, p. 275
Stream FeaturesStream Features
► LoadLoad Sediment load (boulders, gravel, sand), also called bed Sediment load (boulders, gravel, sand), also called bed
loadload Suspended load (silt and clay) high energy events can Suspended load (silt and clay) high energy events can
carry sand and gravelcarry sand and gravel Dissolved load (dissolved minerals)Dissolved load (dissolved minerals)
► Primary and Secondary ChannelPrimary and Secondary Channel► Flood PlainFlood Plain► Cut Bank vs. Point BarCut Bank vs. Point Bar► Meanders and Ox-Bow LakesMeanders and Ox-Bow Lakes► DeltaDelta► Alluvial Fan (intermittent stream)Alluvial Fan (intermittent stream)
Stream LoadStream Load
http://www.indiana.edu/~g103/G103/week6/wk6.html
Fig. 12-7a, p. 278
Braided Stream, New Zealand
Can be caused by additional influx of sediments or a change in topography causing a lowering of the stream gradient
Fig. 12-7a, p. 278Braided stream, Chester, California
Fig. 12-11, p. 281
Primary and Secondary Channels of a StreamPrimary Channel
Secondary Channel
During a flood, the secondary channel begins to fill
Fig. 12-12a, p. 281
Fig. 12-12b, p. 281
Fig. 12-11b, p. 281
Fig. 12-11c, p. 281
Fig. 12-11d, p. 281
Fig. 12-9, p. 279
Fig. 12-9a, p. 279
Fig. 12-9b, p. 279
Fig. 12-6a, p. 277
Sediment load added by erosion at the river bank
Cut Bank
Fig. 12-9c, p. 279
Fig. 12-11a, p. 281
Cut Bank Point Bar
Crossbedding
Fig. 12-8, p. 278
Meandering Stream, low stream gradient, Laxa River, Iceland
Note preliminary formation of an ox-bow lake
Fig. 12-10, p. 280
Fig. 12-10a, p. 280
Fig. 12-10b, p. 280
Fig. 12-10c, p. 280
Fig. 12-10d, p. 280
Fig. 12-23, p. 291Incised meanders—area must have undergone some uplift
Fig. 12-13a, p. 282
Delta
Velocity of the stream is reduced at the mouth of the river.
The sediment and suspended load begins to settle out
Sorting: coarser grains nearer the mouth, finer grains further away
Fig. 12-13b, p. 282
Topset Beds
Foreset Beds
Bottomset Beds
Delta Deposition Pattern
Fig. 12-13c, p. 282
Mississippi River Delta— birds foot delta
Fig. 12-14, p. 283
Alluvial Fans (much coarser-grained than deltas)
Form when heavy rainstorms occur in sparsely-vegetated semi-arid regions.
Fig. 12-14a, p. 283
Fig. 12-14b, p. 283
Fig. 12-21, p. 290
Idealized Stages in the Development of a Stream.
Fig. 12-21a, p. 290
Younger, steeper gradient
Downcutting erosion
Fig. 12-21b, p. 290
Intermediate gradient
Fig. 12-21c, p. 290
Low gradient, mature stream approaching base level
Fig. 12-22, p. 291
Eventually, downcutting will lead to the formation of terraces (which are erosional remnants of floodplains.
Fig. 12-22a, p. 291
Fig. 12-22b, p. 291
Fig. 12-22c, p. 291
Fig. 12-22d, p. 291
Fig. 12-15, p. 285
Drainage Basin
Fig. 12-15a, p. 285Small scale drainage basins follow topography
Divide
Fig. 12-15b, p. 285
Wabash River Drainage Basin
Fig. 12-16, p. 286
Stream Drainage Patterns
Fig. 12-16a, p. 286
Dendritic
Fig. 12-16b, p. 286
Rectangular—controlled by solution joints
Fig. 12-16c, p. 286
Trellis
Fig. 12-16d, p. 286
Radial
Fig. 12-16e, p. 286
Deranged—swampy regions
Fig. 12-17, p. 288
Fig. 12-17a, p. 288
Fig. 12-17b, p. 288
Fig. 12-17c, p. 288
Fig. 12-18, p. 289
Fig. 12-18a, p. 289
Fig. 12-18b, p. 289
Fig. 12-19, p. 289
Fig. 12-19a, p. 289
Fig. 12-19b, p. 289
Fig. 12-20, p. 289
Headward erosion in a valley and stream piracy.
Arrgh!
Fig. 12-20a, p. 289
Fig. 12-20b, p. 289
CHAPTER OUTLINE
• Introduction
• The Hydrologic Cycle
• Running Water
• GEO-FOCUS 12.1: Dams, Reservoirs, and Hydroelectric Power
• How Running Water Erodes and Transports Sediment
• Deposition by Running Water
• Drainage Basins and Drainage Patterns
• CULTURAL CONNECTIONS: Flood Stories from Around the World
• Base Level
• Graded Streams
• Valley Evolution
• Geo-Recap
CHAPTER OBJECTIVES
1 Running water, one part of the hydrologic cycle, does considerable geologic work. Water is continuously cycled from the oceans to land and back to the oceans.
2 Gradient measures how steep a stream is. Discharge measures the volume of water that passes a given point per unit of time. Discharge, along with velocity, usually increases downstream.
3 Running water transports large quantities of sediment and deposits sediment in or adjacent to braided and meandering rivers.
4 Flooding is a natural part of stream activity that takes place when a channel receives more water than it can handle.
5 Alluvial fans (on land) and deltas (in a standing body of water) are deposited when a stream’s capacity to transport sediment decreases.
6 Rivers and streams continuously adjust to changes. Base level is the elevation below which a stream cannot erode. Waterfalls and lakes are temporary base levels, and the sea is ultimate base level.
CHAPTER OBJECTIVES
7 The concept of a graded stream is an ideal, although many rivers and streams approach the graded condition.
8 Most valleys form and change in response to erosion by running water coupled with other geologic processes such as mass wasting.
Fig. 12-2, p. 270
Available Fresh Water
Fig. 12-5, p. 275
Figure 1b, p. 273
Fig. 12-5a, p. 275
CHAPTER SUMMARY
• Water continuously evaporates from the oceans, rises as water vapor, condenses, and falls as precipitation. About 20% of all precipitation falls on land and eventually returns to the oceans, mostly by surface runoff.
• A channel’s gradient varies from steep in its upper reaches to gentle in its lower reaches.
• Flow velocity and discharge are related, so a change in one results in a change in the other. Velocity and discharge increase downstream in most rivers and streams.
• Running water erodes by hydraulic action, abrasion, and dissolution of soluble rocks.
• The larger particles transported by running water move as bed load, whereas the smallest particles move as suspended load. Rivers and streams also transport a dissolved load of materials in solution.
• Braided streams have complex, multiple, intertwining channels. Their deposits consist mostly of sand and gravel.
CHAPTER SUMMARY
• Meandering streams have a single, sinuous channel in which point bars of sand or gravel are deposited. Cutoff meanders known as oxbow lakes eventually fill with fine-grained sediments and organic matter.
• Floodplain deposits might consist of a succession of point bars deposited by a migrating channel, or mud deposited by water carried into the floodplain during floods.
• Deltas form where a river or stream enters a standing body of water and deposits its sediment load. Small deltas in lakes commonly have a three-part division of bottomset, foreset, and topset beds, but marine deltas are larger, more complex, and more important economically.
• In arid and semiarid regions where a river or stream flows from a mountain canyon onto adjacent lowlands, a deposit known as an alluvial fan accumulates. Alluvial fans consist of stream-deposited sand and gravel and/or mudflow deposits.
• Rivers and streams along with their tributaries carry runoff from areas known as drainage basins, which are separated by divides.
CHAPTER SUMMARY
• Sea level is ultimate base level, the lowest level to which rivers and streams can erode. Local or temporary base levels are lakes, other rivers or streams, or particularly resistant rocks.
• Rivers and streams tend to eliminate irregularities in their channels so that they develop a smooth, concave profile of equilibrium. These so-called graded streams approach this ideal condition only temporarily.
• Valleys develop and evolve by several processes, including downcutting, lateral erosion, headward erosion, stream piracy, and mass wasting.
• The formation of a floodplain, followed by renewed downcutting by a stream, leaves remnants of the older floodplain at higher levels known as stream terraces.
Fig. 12-6b, p. 277
Mass wasting
Sediment added to stream by mechanical weathering and erosion
Fig. 12-6c, p. 277
Pothole formed by cutting power of falling water and abrasion from rock inside the pothole.
Fig. 12-6d, p. 277
Note roundness of the boulders inside the pothole.
Stream DepositsStream Deposits
► LeveeLevee► Flood plain depositsFlood plain deposits► Braided streams (sediment load is greater Braided streams (sediment load is greater
than velocity can move)than velocity can move)► Deltas (velocity decreases as stream enters Deltas (velocity decreases as stream enters
larger body of water-not channel flow, larger body of water-not channel flow, allowing for deposition)allowing for deposition)
► Alluvial fans (“semiarid delta”)Alluvial fans (“semiarid delta”)
Stream GradientStream Gradient
►Slope of the channel as it flows downhill.Slope of the channel as it flows downhill.►Steeper gradient in upper reaches, near Steeper gradient in upper reaches, near
headwaters, headwaters, ►Stream gradient gets progressively Stream gradient gets progressively
smaller towards base level.smaller towards base level.►Steep gradient streams are generally Steep gradient streams are generally
straight and cut downward.straight and cut downward.►Shallow gradient streams meander and Shallow gradient streams meander and
generally cut sideways (laterally).generally cut sideways (laterally).