22-2 Pollution of Freshwater Streams

What Are the Water Pollution Problems of Streams? Pollution Overload and Low Flow

Rivers and other flowing streams can recover rap- idly from moderate levels of degradable, oxygen- demanding wastes and excess heat through a combination of dilution and biodegradation of such wastes by bacteria. But this natural recovery process does not work when streams are overloaded with pollutants or when drought, damming, or water diversion for agriculture and industry reduce their flows. Also, these natural dilution and biodegradation processes do not eliminate slowly degradable and nondegradable pollutants.

In a flowing stream, the breakdown of degradable wastes by bacteria depletes dissolved oxygen and creates an oxygen sag curve. This reduces or eliminates populations of organisms with high oxygen requirements until the stream is cleansed of wastes

What Have Developed Countries Done to Reduce Stream Pollution? Good and Bad News

Water pollution control laws enacted in the 1970s have greatly increased the number and quality of wastewater treatment plants in Canada and most other developed countries. Such laws also require industries to reduce or eliminate point-source discharges into surface waters.

These efforts have enabled Canada to hold the line against increased pollution by disease-causing agents and oxygen-demanding wastes in most of its streams. This is an impressive accomplishment given the rise in the country’s industrial activity, resource consumption, and population since pas- sage of these laws.

What Have Developing Countries Done to Reduce Stream Pollution? Little Progress

Available data indicate that stream pollution from discharges of untreated sewage and industrial wastes is a serious and growing problem in most developing countries. According to a report by the World Commission on Water in the 21st Century, half of the world’s 500 major rivers are heavily polluted, most of them running through developing countries. Most of these countries cannot afford to build waste treatment plants and do not have or do not enforce laws for controlling water pollution.