Siltation

The process of erosion in which portions of soil from the banks wash into the river. Silt covers macro invertebrates, thus killing them. The single most important cause of siltation is the removal of trees from along the banks of the river.

Links
  http://www.epa.gov/305b/2000report/chp2.pdf
http://www.trout.forprod.vt.edu/fish_res/pr_area2.htm