Water pollution occurs when substances are released into a body of water, where they become dissolved or suspended or deposited on the bottom, accumulating to the extent that they overwhelm the body of water's capacity to absorb, breakdown, or recycle them, and thus interfere with the functioning of aquatic ecosystems.

Contributions to water pollution include substances drawn from the air (such as acid rain), silt from soil erosion, chemical fertilizers and pesticides, runoff from septic tanks, outflow from livestock feedlots, chemical wastes from industries, and sewage and other urban wastes. A community far upstream in a watershed may thus receive relatively clean water, whereas one farther downstream receives a partly diluted mixture of urban, industrial, and rural wastes.

When organic matter exceeds the capacity of microorganisms in the water to break it down and recycle it, the excess of nutrients in such matter encourages algal water blooms. When these algae die, their remains add further to the organic wastes already in the water, and eventually the water becomes deficient in oxygen. Organisms that do not require oxygen then attack the organic wastes, releasing gases such as methane and hydrogen sulfide, which are harmful to the oxygen-requiring forms of life. The result is a foul-smelling, waste-filled body of water.