Confluence of two streams, with water from one stream muddy from sediment pollutionSediment is composed of loose particles of sand, clay, silt, and other substances. It comes from eroding soil and runoff. Much of the sediment in Minnesota lakes and rivers is contaminated by pollutants, particularly phosphorus.

Sediment contributes to turbidity — cloudy water that is harmful to fish and plant life — and, in large quantities, can fill in bodies of water. For instance, the upper seven miles of Lake Pepin on the Mississippi River could be completely filled in with sediment deposits over the next 100 years if nothing is done to remedy the problem.