Biochemical oxygen demand is a measure of the quality of oxygen used by these microorganisms in the aerobic oxidation of organic matter. When aquatic plants die, they are fed upon by aerobic bacteria. The input of nutrients into a river, such as nitrates and phosphates, stimulate plant growth. Eventually, more plant growth leads to more plant decay. Nutrients, then, can be a prime contributor to high biochemical oxygen demand in rivers.
Impounded river reaches also collect organic wastes from upriver that settle in quieter water.
The bacteria that feed on this organic waste consume oxygen. There are natural sources of organic material which enter lakes and rivers from swamps, bogs, and vegetation along the water, particularly leaf fall. There are also human sources of organic material. When these are identifiable points of discharge into rivers and lakes, they are called point sources. In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live.