Chapter 1313.3 River Valleys

River ValleysRain may form a small

valley in soft bedrock on hill slope via headwater erosion

Form in wet areasStream cuts a gully

and may disappearEach rain increases

LengthWidthDepth of River

Headward Erosion

is demonstrated in this 1906 photo taken near Mount Tamalpais in Marin County, California. Groundwater sapping is causing this gully to lengthen up the slope.

Permanent RiverStream cuts down

deep enough to become permanent

If tributaries become permanent, then river system is born

Canyons and V shaped ValleysRiver Valley Canyon

Canyons, Gorges and ChasmsVery Steep almost

Vertical sidesRegions of less rainfallForm 2 ways:

Lack of rainfallErosion resistant

rocksFormation Factors:

Type of rockAmount of waterSediment in riverClimate of region

Gorge

Young ValleyV-shaped ValleysYoung RiverRiver cuts down into

channelUpper valley walls

widened by erosion

Old ValleyU-Shaped ValleyLess erosion of bedMore erosion of

channel sidesWide valley, broad

floor, gently sloping sides

Base LevelStream cannot erode

bed lower than body of water it flows into

Sea level is ultimate base level

Stream Piracy or stream capture

Headward erosion cuts through divide

One river captures or “pirates” the headwaters of another river

River grows largerExpands drainage basinFormation process of

great river systemsMississippi, Nile,

Ganges, etc.

RapidsSteep riverbedWater velocity is fastForms white-water

rapids

Rapids

WaterfallWater flows over

hard rockErodes softer rock

belowFalls are temporary

Undermines rock above leaving overhang

Overhang breaks offWaterfall recedes

upstream

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