The second type of river, the “alluvial,” is encountered in places less steep, in the bottom of wider valleys, valleys that have flat bottomland (the floodplain) bordering the river. An alluvial river is distinguished by the materials which form its riverbed and riverbanks. These rivers flow in channels constructed from sediments which the river itself has deposited, sediments which were excavated from erosion occurring in channels and valleys further upstream. The distinguishing characteristic of an alluvial river is that it constructs its own channel with materials carried in its current of water. Both erosion and deposition, equally, are processes which dominate the form and evolution of alluvial rivers. And the flat bottomland bordering the river, its floodplain, consists of soils derived from sediments deposited by the river over thousands of years. These rivers are called “alluvial” because they are constructed from water-borne, or“alluvial,” sediments.
Stages in the evolution of a river cross section in response to artificially straightening the channel (stage 2). Straightening results in a shorter, steeper channel. Because it is steeper, the erosive forces are greater, and the channel must go through stages of adjustment in order to bring erosion back into balance with deposition. A very similar sequence of adjustment happens when erosion is increased by enhanced water runoff, as from urbanization, forest fires, etc. The thick arrows show direction of change for the streambed and banks. The height of the top of bank, h, changes from typical, stable floodplain or “bankfull”height, to heights too large to remain stable, then returns to bankfull, as a new floodplain develops. The old floodplain is called a terrace, and is flooded infrequently, only by the largest floods. Adapted from Cramer, Michelle L. (managing editor), 2012, Stream Habitat Restoration Guidelines, WDFW, Olympia, WA.