For example, if a river or stream flows through an agricultural area, it can pick up fertilizer, manure, and pesticides from farming operations that run off the land after a rainstorm. As it passes urbanized and suburbanized areas, it might gather fertilizers that wash off lawns, untreated sewage from failing septic tanks, wastewater discharges from industrial facilities, sediment from construction sites, and runoff from impervious surfaces like parking lots. Upon reaching the coast, the stream or river can be affected by commercial and recreational boating, discharges from industrial and municipal facilities, and recreational activities on beaches. The result can be muddy waters that smother organisms living on the bottom, decrease the amount of light reaching the sea grass beds, and clog fish gills. Some kinds of pollutants can bind to sediment and flow with it to coastal waters.
Excess nutrients can also wash off the land when it rains and end up in coastal waters.
Sources of excess nutrients include lawn fertilizers, pet and farm animal waste, decaying plant material, failing septic tanks, atmospheric deposition, and inefficient sewage treatment plants. The loss of wetlands in many watersheds has reduced the ability of nature to process these nutrients before they enter rivers, streams, and ultimately estuaries. When the algae die off, the decaying process uses up the oxygen in the water, leaving little, if any, for fish and other aquatic organisms. Toxic substances , such as pesticides from lawns, gardens, and farms, and lead, oils, and greases deposited on roads from cars and trucks, can all run off the land with rainfall and snowmelt. Industrial plants and municipal wastewater treatment plants can also contribute to the amount of toxic substances entering streams and rivers and ultimately lakes, estuaries, and coastal waters.