The US Environmental Protection Agency lists fine sediment – primarily silt and clay – as the most widespread river pollutant in the contiguous United States. As noted in the earlier description of the effects of changing land cover, excess fine sediment clogs the pores between sand and cobble on the streambed, limiting hyporheic exchange and the interstitial space available for microbial communities, larval insects, and fish eggs. When fine sediment remains suspended it reduces the clarity of the water and can limit photosynthesis by aquatic plants and algae attached to the streambed. In high concentrations, suspended sediment can clog the gills of amphibians and fish. Silt and clay can also stimulate the growth of cyanobacteria that can produce chemicals toxic to other organisms. Continuing large inputs of fine sediment can cause so much sedimentation in a river corridor that backwater lakes and floodplain wetlands completely fill, destroying habitat for fish, amphibians, waterfowl, and other organisms. Sediment deposited in ponded areas can also release methane, a greenhouse gas, as organic matter within the sediment decays. Silt and clay also carry adsorbed contaminants.

Although fine sediment can enter rivers through bank erosion, the majority comes from erosion of adjacent uplands, typically as a result of agriculture and removal of native land cover. Globally, the cumulative effect of human activities now moves more sediment than rivers, glaciers, and other natural processes, and much of this sediment makes its way into river corridors.

Bhowmik, N.G. and M. Demissie. 1989. Sedimentation in the Illinois River Valley and backwater lakes. Journal of Hydrology, 105, 187-195.

Hooke, R. LeB. 2000. On the history of humans as geomorphic agents. Geology, 28, 843-846.