Chapter 13 13.3 River Valleys

River Valleys Rain may form a small valley in soft bedrock on hill slope via headwater erosion Form in wet areas Stream cuts a gully and may disappear Each rain increases Length Width Depth 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 River Stream cuts down deep enough to become permanent If tributaries become permanent, then river system is born

Canyons and V shaped Valleys River Valley Canyon

Canyons, Gorges and Chasms Very Steep almost Vertical sides Regions of less rainfall Form 2 ways: Lack of rainfall Erosion resistant rocks Formation Factors: Type of rock Amount of water Sediment in river Climate of region

Gorge

Young Valley V-shaped Valleys Young River River cuts down into channel Upper valley walls widened by erosion

Old Valley U-Shaped Valley Less erosion of bed More erosion of channel sides Wide valley, broad floor, gently sloping sides

Base Level Stream 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 larger Expands drainage basin Formation process of great river systems Mississippi, Nile, Ganges, etc.

Rapids Steep riverbed Water velocity is fast Forms white-water rapids

Rapids

Waterfall Water flows over hard rock Erodes softer rock below Falls are temporary Undermines rock above leaving overhang Overhang breaks off Waterfall recedes upstream

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