When organic matter decomposes, it is fed upon aerobic bacteria, the organic material is broken down and oxidized. BOD is the measure of the quanitity of oxygen used by the microorganisims in the aerobic oxidation.

Nutrients affect the levels. This is because the more nutrients there are in the water, the more stimulation of plant growth and as plants die, they are fed upon by aerobic bacteria.

In rivers with high BOD levels, much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the needed oxygen.