River pollution comes from a variety of different sources, including agricultural operations, industrial discharge, wastewater treatment plants and storm water runoff, that carry pollutants into waterways. Sources are classified as point sources if the pollution comes from a single, identifiable source and nonpoint sources if the pollution is the result of runoff over a wide area. Since rivers draw from many sources, they may contain many different pollutants.
Point-source pollution is the easiest type of river pollution to correct, as it can be traced to a single source. An industrial operation may accidentally discharge untreated pollutants into a river, or a flood at a wastewater treatment plant may cause sewage to flow into a waterway. These spills can be massive and release large amounts of pollutants over a short period of time, but once the problem is corrected, the river can eventually recover. Nonpoint-source pollution is much more difficult to deal with. As rain or floodwater flows over land, it picks up any contaminants present. Oil and grease from roadways, excess nitrogen and pesticides from farmland, heavy metals from mining and scrapping operations and loose sediment may find their way into the river, carried through the water cycle by runoff.