The dynamic concept of sediment sources

Using 200 upland sample areas and 23 valley cross‐sections, a study was made of rates of sediment production and sediment sources from the headwaters of the Sangamon River of Illinois to its mouth. It was found that as the size of drainage area increases, the upland sources of sediment, such as sheet and gully erosion, become less important, while the bottomland sources, such as streambank erosion, floodplain scour, and valley trenching, become constantly more important. The writer maintains that this dynamic concept of changing sediment sources with changing size of drainage area applies not only to the Sangamon River Watershed, but to any watershed. It was also found that the annual rate of sediment production in the Sangamon watershed decreases from an average of 2940 tons/sq mi of drainage area in the headwaters to 746 tons/sq mi at the mouth. This agrees with previous studies by the writer, which show that generally rates of sediment production decrease with increasing size of drainage area, for reasons which are set forth in the text. The dynamic concept of sediment sources is compared with the previously widely used static concept based upon total erosion figures in a watershed, and the errors involved in the older concept exposed. One of the more important conclusions drawn from this study is that streambank and other forms of bottomland erosion have not been given enough attention by the various agencies concerned with their control, in comparison with their importance when viewed in the light of the dynamic concept.