Landforms of Alluvial Rivers
Rivers that have reached base level develop broad valleys by erosion caused by meandering channels. The stream channel cuts through and redistributes its sediment or alluvium that lines the area bordering the stream.
Figure 18.34 Alluvial Stream Features
Figure 18.35 An alluvial river meanders through its floodplain (Courtesy USGS DDS21)
A floodplain is the relatively flat area that borders a stream which is periodically inundated with water during high flow periods. When excess runoff causes the stream discharge to increase beyond the capacity of the channel, water spills out onto the floodplain. Increasing the cross-sectional area of stream flow causes a decrease in stream velocity. The resulting decrease in velocity causes sediment to deposit as alluvium on the floodplain . These alluvial deposits are often rich in nutrients and thus naturally fertilize floodplain soils. Floodplain agriculture has given rise to many of the great world civilizations.
A natural levee is an narrow ridge of alluvium deposited at the side of the channel. During high discharge periods when the stream floods, coarse sediment settles out near the stream channel and grades to finer material further away. The over bank deposits of alluvium are often rich sources of nutrients for soils developed on the floodplain. Because floodplain soils are usually quite fertile, humans have inhabited them for years. To prevent flooding, artificial levees are built close to the channel, typically higher than natural levees. Confining the flood discharge to a small area increases the velocity of flow. The levees of the Mississippi River increase the flow velocity near the mouth as it enters the Gulf of Mexico. As a result, sediment is shot into the Gulf rather than being deposited near the mouth building the river's famous 'bird's foot' delta.
Figure 18.36 Draining back swamp of Roanoke River Courtesy US FWS (Source)
Back swamps are located some distance away from the stream channel on the floodplain. When water spills over onto the floodplain, the heaviest material drops out first and finest material is carried a greater distance. The fine grained alluvium holds much water and drains rather slowly creating wetland areas. Back swamps are important "sponges" that retain water that might cause severe flooding downstream.
Help keep this site available by donating through PayPal.
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License..