A river or stream is primarily a transport system between the land and the sea. Most of the nutrients in a river come from adjacent terrestrial ecosystems, and relatively little primary production occurs within the streams or rivers itself. The organisms that are characteristic of rivers and streams are especially adapted to feeding on organic ditritus. Some insect larvae, such as caddis flies, attach themselves to rocks and spin nets which catch food particles from the flowing waters. In slow-moving rivers and pools the biological communities are much like those in lakes and ponds. If the load of the organic material reaching a river in runoff from the land is not too great, it will be quickly converted into its inorganic constituents by an array of organisms adapted for this function.
Rivers and streams serve human society in many important ways. They provide water for drinking, irrigation, and industrial use. Rivers are used as disposal systems for sewage and industrial wastes, was well as for means of transportation and recreation. Boats and barges transport the raw materials and products of industry and society along the waterways of the world. In spite of the fact that they make up only a very small percentage of the earth's surface, man uses rivers and streams more intensively than any other natural ecosystem.
Before the industrial revolution our rivers could easily absorb the impact of man without damage to their normal functions. However, huge amounts of water used by modern society, and the heavy burdens of waste that are now emptied into these ecosystems, have placed them in a precarious state. The two categories of waste are those that consist of materials that are normally present in natural ecosystems and those that are not usually found in nature. In the first instance there are organisms in the ecosystem which are adapted to handle these products in reasonable amounts. Human sewage and fertilizers are in this category. As long the human sewage and fertilizers that enter a river do not exceed the river's capacity to use the breakdown products of these materials, the system will not be harmed. When the system is overloaded, pollution results.
Many of the chemicals in industrial wastes are in the second category. There are no organisms capable of breaking down pesticides and many industrial chemicals before they reach toxic concentrations, so these materials are poisons when they enter a natural ecosystem.