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Running Water (Shaping Earth’s Surface,

Part 4)

Science 330 Summer 2005

Hydrologic cycle

The hydrologic cycle is a summary of the circulation of Earth’s water supplyProcesses involved in the hydrologic cycle

PrecipitationEvaporationInfiltrationRunoffTranspiration

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The hydrologic cycle

Sources of Earth’s water

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Running water

Begins as sheetflowInfiltration capacity is controlled by

Intensity and duration of rainfallPrior wetted condition of the soil

Soil textureSlope of the land

Nature of the vegetative cover

Sheetflow develops into tiny channels called rills

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StreamflowTwo types of flow determined primarily by velocity

Laminar flowTurbulent flow

Factors that determine velocityGradient, or slopeChannel characteristics including shape, size, and roughness

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StreamflowFactors that determine velocity

Discharge – the volume of water moving past a given point in a certain amount of time

Changes from upstream to downstreamProfile

Cross-sectional view of a streamViewed from the head (headwaters or source) to the mouth of a stream

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Changes from upstream to downstreamProfile

Profile is a smooth curveGradient decreases downstream

Factors that increase downstreamVelocity

DischargeChannel size

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Longitudinal profile of a stream

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Changes from upstream to downstreamFactors that decrease downstream

GradientChannel roughness

Base level and graded streamsBase level is the lowest point to which a stream can erode

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Base level and graded streamsTwo general types of base level

Ultimate (sea level)Local or temporary

Changing conditions causes readjustment of stream activities

Raising base level causes depositionLowering base level causes erosion

Adjustment of base level to changing conditions

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A waterfall is an example of a local base level

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Stream erosionLifting loosely consolidated particles by

AbrasionDissolution

Stronger currents lift particles more effectively

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Transport of sediment by streamsTransported material is called the stream’s loadTypes of load

Dissolved loadSuspended loadBed load

Capacity – the maximum load a stream can transport

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CompetenceIndicates the maximum particle size a stream can transportDetermined by the stream’s velocity

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Deposition of sediment by a streamCaused by a decrease in velocity

Competence is reducedSediment begins to drop out

Stream sedimentsGenerally well sortedStream sediments are known as alluvium

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Deposition of sediment by a streamChannel deposits

BarsBraided streamsDeltas

Floodplain depositsNatural levees – form parallel to the stream channel by successive floods over many years

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Formation of natural levees

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Deposition of sediment by a streamFloodplain deposits

Back swampsYazoo tributaries

Alluvial fansDevelop where a high-gradient stream leaves a narrow valleySlopes outward in a broad arc

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Deposition of sediment by a streamDeltas

Forms when a stream enters an ocean or lakeConsists of three types of beds

Foreset bedsTopset bedsBottomset beds

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Stream valleysThe most common landforms on Earth’s surfaceTwo general types of stream valleys

Narrow valleysV-shaped Downcutting toward base levelFeatures often include rapids and waterfalls

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Stream valleysTwo general types of stream valleys

Wide valleysStream is near base levelDownward erosion is less dominantStream energy is directed from side to side forming a floodplain

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Stream valleysFeatures of wide valleys often include

FloodplainsErosional floodplainsDepositional floodplains

MeandersCut bank and point barCutoffs and oxbow lakes

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Erosion and deposition along a meandering stream

A meander loop on the Colorado River

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Incised meanders and stream terracesIncised meanders

Meanders in steep, narrow valleysCaused by a drop in base level or uplift of the region

TerracesRemnants of a former floodplainRiver has adjusted to a relative drop in base level by downcutting

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Drainage networksLand area that contributes water to the stream is the drainage basinImaginary line separating one basin from another is called a divide

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Drainage basin

Drainage basin of the Mississippi River

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Drainage patternPattern of the interconnected network of streams in an areaCommon drainage patterns

DendriticRadialRectangularTrellis

Drainage patterns

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Headward erosion and stream piracyA stream can lengthen its course by either

Building a deltaHeadward erosion

Headward erosion may result in stream piracy – the diversion of the drainage of one stream into another

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Formation of a water gapA water gap is a notch where a river cuts through a ridge that lies in its pathTwo possible methods of formation

Antecedent stream – stream existed before the ridge was formedSuperposed stream – stream let down upon a preexisting structure

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Floods and flood controlFloods are the most common and most destructive geologic hazardCauses of flooding

Result from naturally occurring and human-induced factors

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Floods and flood controlTypes of floods

Regional floodsFlash floodsIce-jam floodsDam failure

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Floods and flood controlFlood control

Engineering effortsArtificial leveesFlood-control damsChannelization

Nonstructural approach through sound floodplain management

End of Chapter 10