Streams and FloodsStreams and Floods

Physical Geology, Chapter 10Physical Geology, Chapter 10

Chp. 10Chp. 10Running WaterRunning Water

• Running waterRunning water is the most is the most important geologic agent of eroding, important geologic agent of eroding, transporting and depositing sedimenttransporting and depositing sediment

Nearly every landscape on Earth Nearly every landscape on Earth shows the results of stream erosion shows the results of stream erosion

or depositionor deposition

Hydrologic CycleHydrologic Cycle• Hydrologic cycleHydrologic cycle - the movement and - the movement and

interchange of water between the sea, air, and interchange of water between the sea, air, and landland

Evaporation Evaporation

CondensationCondensation

Precipitation Precipitation

TranspirationTranspiration

RunoffRunoff

InfiltrationInfiltration

StreamsStreams• StreamStream -- a body of running water, confined to a a body of running water, confined to a

channel, that runs downhill under the influence of channel, that runs downhill under the influence of gravitygravity– HeadwatersHeadwaters

• upper part of stream near its source in the mountainsupper part of stream near its source in the mountains

– MouthMouth • place where a stream enters sea, lake or larger streamplace where a stream enters sea, lake or larger stream

– ChannelChannel • a long, narrow depression eroded by a stream into rock or a long, narrow depression eroded by a stream into rock or

sedimentsediment– Stream Stream banksbanks - sides of channel - sides of channel– StreambedStreambed - bottom of the channel - bottom of the channel

– FloodplainFloodplain • flat valley floor composed of sediment deposited by the flat valley floor composed of sediment deposited by the

streamstream

Drainage BasinsDrainage Basins

• Drainage basinDrainage basin - - the total area drained by the total area drained by a stream and its tributariesa stream and its tributaries

• TributaryTributary - -a small stream flowing into a larger a small stream flowing into a larger oneone

• Can be outlined on a map by drawing a line around the Can be outlined on a map by drawing a line around the region drained by all of the tributaries of a streamregion drained by all of the tributaries of a stream

• DivideDivide - ridge or strip of high ground that divides one - ridge or strip of high ground that divides one drainage basin from anotherdrainage basin from another

•Continental divideContinental divide separates the streams that separates the streams that flow into the Pacific from those that flow into the flow into the Pacific from those that flow into the atlantic and Gulf of Mexicoatlantic and Gulf of Mexico

Drainage PatternsDrainage Patterns• Drainage patternDrainage pattern

– the arrangement, in map view, of a stream and its the arrangement, in map view, of a stream and its tributariestributaries

– Most tributaries join the main stream at an Most tributaries join the main stream at an acute angleacute angle, , forming a V or Y pointing downstreamforming a V or Y pointing downstream

1. Dendritic1. Dendritic - drainage pattern resembling the branches of - drainage pattern resembling the branches of a treea tree

2. Radial pattern2. Radial pattern - streams diverge outward like the spokes - streams diverge outward like the spokes of a wheelof a wheel• Typically form on high conical mountainsTypically form on high conical mountains

3. Rectangular pattern3. Rectangular pattern - - tributaries have frequent 90° tributaries have frequent 90° bends and join other streams at right anglesbends and join other streams at right angles

4. Trellis pattern4. Trellis pattern - parallel streams with short tributaries - parallel streams with short tributaries meeting at right anglesmeeting at right angles

Stream ErosionStream Erosion

• Stream erosion and deposition are controlled Stream erosion and deposition are controlled mainly by a river’s mainly by a river’s velocityvelocity and and dischargedischarge– Stream velocityStream velocity is largely controlled by stream is largely controlled by stream

gradientgradient (slope), (slope), channel shapechannel shape and and channel channel roughnessroughness

• Stream reaches maximum velocity near Stream reaches maximum velocity near centercenter of channelof channel

– Floods involve an Floods involve an increase in velocity and total increase in velocity and total dischargedischarge (volume of water passing a particular (volume of water passing a particular point in a stream over time)point in a stream over time)

• Higher flow velocitiesHigher flow velocities in a stream allow it to in a stream allow it to

transport transport coarser sedimentscoarser sediments

Stream Erosion and Stream Erosion and DepositionDeposition• StreamStream gradientgradient is the downhill slope of the is the downhill slope of the

streambedstreambed– Typically measured in feet per mile in the Typically measured in feet per mile in the

U.S., and in meters per kilometer elsewhereU.S., and in meters per kilometer elsewhere– Usually decreases downstreamUsually decreases downstream

• Channel shape and roughnessChannel shape and roughness – Both effect stream velocity due to drag Both effect stream velocity due to drag – Narrower, deeperNarrower, deeper channels allow channels allow faster flowfaster flow– SmootherSmoother channels allow channels allow faster flowfaster flow– Wider, shallower Wider, shallower channels channels decrease flow decrease flow

speedspeed– Rougher Rougher channels channels decrease flow speeddecrease flow speed

• Stream Stream dischargedischarge is the volume of water is the volume of water flowing past a given point in a unit of timeflowing past a given point in a unit of time

Stream ErosionStream Erosion• Streams cut their own valleys, deepening and Streams cut their own valleys, deepening and

widening them over time and carrying away the widening them over time and carrying away the sedimentsediment

• Stream erosion occurs by three mechanisms:Stream erosion occurs by three mechanisms:

hydraulic action, solution, hydraulic action, solution, and and abrasionabrasion

1.Hydraulic action1.Hydraulic action - ability of flowing water to - ability of flowing water to pick up and move rock and sedimentpick up and move rock and sediment

2.Solution 2.Solution - dissolving of rocks (e.g., limestone)- dissolving of rocks (e.g., limestone)

3.Abrasion3.Abrasion - grinding away of stream channel by - grinding away of stream channel by the friction and impact of the sediment loadthe friction and impact of the sediment load

• PotholesPotholes are eroded into streambed by the are eroded into streambed by the abrasive action of the sediment load in the streamabrasive action of the sediment load in the stream

Sediment Sediment TransportationTransportation

• Sediment load transported by a stream can be Sediment load transported by a stream can be subdivided intosubdivided into bed loadbed load,, suspended load suspended load, and, and dissolved loaddissolved load

1.Bed Load1.Bed Load - large or heavy particles that travel on - large or heavy particles that travel on the streambedthe streambed

A.Traction loadA.Traction load - large particles that travel along - large particles that travel along the streambed by rolling, sliding or draggingthe streambed by rolling, sliding or dragging

B.Saltation loadB.Saltation load - medium particles (typically - medium particles (typically sand-sized) that travel downstream by bouncing sand-sized) that travel downstream by bouncing along - sometimes in contact with the streambed along - sometimes in contact with the streambed and sometimes suspended in the flowing waterand sometimes suspended in the flowing water

Sediment Transportation Sediment Transportation ContinuedContinued

2.Suspended load2.Suspended load - sediment that is small/light - sediment that is small/light enough to remain above the stream bottom by enough to remain above the stream bottom by turbulent flow for an indefinite period of timeturbulent flow for an indefinite period of time

3.Dissolved load3.Dissolved load - dissolved ions produced by - dissolved ions produced by chemical weathering of soluble minerals chemical weathering of soluble minerals upstreamupstream

Sediment DepositionSediment Deposition

• BarsBars - ridges of sediment (usually sand or gravel) - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a deposited in the middle or along the sides of a streamstream

• Braided streamsBraided streams contain an overabundance of contain an overabundance of sediment deposited as numerous bars around which sediment deposited as numerous bars around which water flows in a system of highly interconnected water flows in a system of highly interconnected rivuletsrivulets

• Meandering streamsMeandering streams flow faster along the flow faster along the outside of bends (meanders) and more slowly along outside of bends (meanders) and more slowly along the inside, during floods.the inside, during floods.

Sediment DepositionSediment Deposition

• Floodplains Floodplains are broad strips of land are broad strips of land built up by sedimentation on either side built up by sedimentation on either side of a stream channelof a stream channel

– Floodplain sedimentsFloodplain sediments are left behind as flood are left behind as flood waters slow and recede at the end of flood waters slow and recede at the end of flood events.events.

– The Main channel has slightly raised banks with The Main channel has slightly raised banks with respect to the floodplain known as respect to the floodplain known as natural natural leveeslevees

Sediment DepositionSediment Deposition

• Deltas Deltas - a body of sediment deposited at - a body of sediment deposited at the mouth of a river when the flow velocity the mouth of a river when the flow velocity decreasesdecreases

– Shape of a delta will depend on whether its Shape of a delta will depend on whether its growth and evolution are growth and evolution are wave-dominatedwave-dominated, ,

tide-dominatedtide-dominated, or , or stream-stream-dominateddominated

Sediment DepositionSediment Deposition

• Alluvial fansAlluvial fans - large, fan- or cone-shaped - large, fan- or cone-shaped piles of sediment that usually form where a piles of sediment that usually form where a stream’s velocity decreases as it emerges stream’s velocity decreases as it emerges from a narrow mountain canyon onto a flat from a narrow mountain canyon onto a flat plainplain

– Particularly well-developed in desert Particularly well-developed in desert regions, such as the southwestern U.S.regions, such as the southwestern U.S.

– Larger fans show grading from large Larger fans show grading from large sediments nearest the mountains and finer sediments nearest the mountains and finer sediments deposited progressively farther sediments deposited progressively farther awayaway

FloodingFlooding• When water levels rise and overtop the banks When water levels rise and overtop the banks

of a river, of a river, floodingflooding occurs occurs– Natural process on Natural process on allall rivers rivers – Described by Described by recurrence intervalsrecurrence intervals

•A A 100-year flood100-year flood is, on average, the size is, on average, the size of the largest flood within a 100-year of the largest flood within a 100-year period of timeperiod of time

– Can cause Can cause great damagegreat damage in heavily in heavily populated areaspopulated areas

– High velocity and large volume of water in a High velocity and large volume of water in a flood causes flood causes flood erosionflood erosion

– Slowing of waters as flood waters recede Slowing of waters as flood waters recede causes causes flood depositsflood deposits (usually of silt or clay- (usually of silt or clay-sized particles) to be laid down in the sized particles) to be laid down in the floodplainfloodplain

• Urban floodingUrban flooding– Paved areas and storm sewers increase Paved areas and storm sewers increase

runoff by inhibiting infiltrationrunoff by inhibiting infiltration– Rapid delivery of water to streams increases Rapid delivery of water to streams increases

peak discharge and hastens occurrence of peak discharge and hastens occurrence of floodflood

• Flash floodsFlash floods– Local, sudden floods of large volume and Local, sudden floods of large volume and

short durationshort duration– Typically triggered by heavy thunderstormsTypically triggered by heavy thunderstorms

• Flood controlFlood control– DamsDams designed to trap flood waters in designed to trap flood waters in

reservoirs upstream and release it gradually reservoirs upstream and release it gradually over time.over time.

– Artificial leveesArtificial levees designed to increase designed to increase capacity of river channelcapacity of river channel• Works well until stream levels overtop artificially Works well until stream levels overtop artificially

raised levees, leading to extremely rapid flooding and raised levees, leading to extremely rapid flooding and erosion than would have occurred otherwiseerosion than would have occurred otherwise

– Wise land-use planningWise land-use planning,, including including prevention of building within 100-year prevention of building within 100-year floodplains, is most effectivefloodplains, is most effective

Stream Valley Stream Valley DevelopmentDevelopment

• DowncuttingDowncutting– Process of deepening a valley by erosion of the Process of deepening a valley by erosion of the

streambedstreambed

•V-shaped valleysV-shaped valleys typically form from typically form from downcutting of river channel combined with downcutting of river channel combined with mass wasting and sheet erosion of valley wallsmass wasting and sheet erosion of valley walls

•Streams cannot erode below the elevation of Streams cannot erode below the elevation of the mouth of the stream (the the mouth of the stream (the base levelbase level))

•Base level can be sea level, a lake, or the Base level can be sea level, a lake, or the bottom of a closed basin (like Death Valley, bottom of a closed basin (like Death Valley, CA)CA)

• Graded streamsGraded streams– Characteristic concave-up longitudinal profileCharacteristic concave-up longitudinal profile– Rapids and waterfalls have been smoothed Rapids and waterfalls have been smoothed

out by prior downcuttingout by prior downcutting– Delicate balance between available sediment Delicate balance between available sediment

load and transport capacityload and transport capacity

• Lateral erosionLateral erosion widens stream valleys by widens stream valleys by undercutting of stream banks and valley walls undercutting of stream banks and valley walls as stream swings from side to side across the as stream swings from side to side across the valley floorvalley floor

• Headward erosionHeadward erosion is the slow uphill is the slow uphill growth of a valley above its original source by growth of a valley above its original source by gullying, mass wasting, and sheet erosiongullying, mass wasting, and sheet erosion

• Stream terracesStream terraces– Step-like landforms found above a stream and its Step-like landforms found above a stream and its

floodplainfloodplain– Occurs when river cuts downward into its own Occurs when river cuts downward into its own

floodplain depositsfloodplain deposits– Represents change from deposition to erosionRepresents change from deposition to erosion– Can be caused by Can be caused by

• rapid upliftrapid uplift, , drops in base leveldrops in base level, or , or climate changesclimate changes

• Incised meandersIncised meanders– Retain their sinuous pattern as they cut vertically Retain their sinuous pattern as they cut vertically

below original levelbelow original level– May be produced by profound base level changes, as May be produced by profound base level changes, as

due to due to rapid upliftrapid uplift

• Superposed streamsSuperposed streams– Rivers superposed onto mountain range from an eroding Rivers superposed onto mountain range from an eroding

floodplain that used to exist above itfloodplain that used to exist above it– Several rivers cutting through the previously partly-buried Several rivers cutting through the previously partly-buried

Appalachians are examples (Potomac, Susquehanna) Appalachians are examples (Potomac, Susquehanna)

Stream Valleys on MarsStream Valleys on Mars• Evidence of different climate in pastEvidence of different climate in past

– Liquid water is not stable on surface of Mars Liquid water is not stable on surface of Mars

under under present conditionspresent conditions

•Too coldToo cold

•Atmospheric pressure too lowAtmospheric pressure too low– Stream channels and terraces suggest Stream channels and terraces suggest long-term long-term

erosionerosion by flowing water by flowing water– Lack of smaller tributaries of larger channels is Lack of smaller tributaries of larger channels is

puzzling, but these do exist for channels puzzling, but these do exist for channels networks in more ancient terrains on Marsnetworks in more ancient terrains on Mars

– Requires Requires warmerwarmer,, wetter Mars wetter Mars in past in past– NASA missions targeting such locationsNASA missions targeting such locations