What Human Activities Cause Low Levels Of Dissolved Oxygen?

Warm water discharged from factories, wastewater treatment plants or power plants reduces dissolved oxygen levels. This is known as thermal pollution.
Warm water with low oxygen levels is found in slow, shallow rivers created by withdrawing water for irrigation or the filling of reservoirs. In 2000, there was concern that irrigation water taken from the Little Bow River might leave fish in a desperate situation during late summer.
Warm water with low oxygen levels can result when vegetation is removed from stream banks during landscaping, logging or clearing farmland. Less vegetation results in less shade from the sun and higher water temperatures result.
Nutrients added to water by fertilizers washing-off fields or added by urban sewage can produce excessive plant or algae growth in rivers. During late summer or winter, decomposing bacteria break down the masses of dying aquatic plants and algae. This decay process consumes large amounts of dissolved oxygen, affecting fish and other pollution sensitive organisms.
Animal and plant waste (pulp, manure, vegetable peels, blood, leaves, grass) entering rivers from pulp mills, feedlots, dairies, food-processing plants, meatpacking plants, forests and lawns create eutrophic or organically enriched conditions. This organic loading may result in low oxygen levels as bacteria decompose the material. When populations of microscopic decomposers rapidly increase, a situation of high biological oxygen demand (B.O.D.) is created. Under extreme conditions of high B.O.D., anaerobic bacteria produce hydrogen sulfide gas with a rotten egg smell.