The organic matter available for decomposition has both natural and human origins. Nutrients are the main culprit for high BOD in river water. Calm stretches of water ways also collect organic wastes that settle out from upstream. Swamps, bogs, and vegetation along the water provide organic matter for decomposition. Human activities can result in point source and nonpoint source contribution.
Urban runoff is a nonpoint source provider.
Rain and melting snow carry sewage form improper sewer connections into storm drains. Pet wastes wash off sidewalks. Nutrients from lawn fertilizers, leaves, grass clippings, and paper from residential areas also find their way to the water. Nitrates, phosphates, and other nutrients are carried from fields by agricultural runoff. Chemical oxidation of sulfides, ferrous ions, and ammonia also consume oxygen in water.