Riverand Stream Ecosystems is very important in the world , The history of man has often been shaped by the rivers that provide water, transportation, and a means of waste disposal. Although the total surface area of rivers and streams is small compared to that of oceans and land mass, rivers are among the most intensely used by man of natural ecosystems. As in the case of estuaries, the need fo ‘’multiple use’’ (as contrasted to a ‘’single use’’ approach to such ecosystems as cropland) demands that the various areas (water supply, waste disposal, fish production, flood control, and so on) be considered together and not as entirely separate problems.

         From the energetic standpoint rivers and streams are incomplete ecosystems; that is, some portion, often a large protion, of the biological energy flow is based on organic matter imported from adjacent terrestrial ecosystems, or sometimes from adjacent lakes. Although streams are naturally adapted waste treatment systems for degradable wastes (recall our frequent comment about ‘’free sewers’’) almost all of the world’s great rivers are severely over loaded with the residues of man’s civilization. As geographer M. G, Wolman (1971) has concluded, ‘’demands on water resources are increasing at a rate that exceeds the rate of installation of waste treatment facilities.’’ This is another one of those ‘’mismatched rates’’ that are at the heart of man’s troubles with his environment. In all parts of the world man has so extensively dammed, diked, and channelized rivers that it is getting hard to find a truly wild river of any size. It is turning out that some of these manipulations bring only temporary or local benefits at great cost, and create additional problems costing still more money to correct (as in the case of some flood control projects). Accordingly, flood damages that used to be considerednatural disaster’s (and therefore, unavoidable) are more and more proving to be man – made disasters (and, therefore, avoidable). In the future, proposed alterations will have to be subjected to a more thorough cost – benefit analysis than was the case in the past. More about this in the next post.

         The stream ecologist finds it convenient to consider flowing water ecosystems under two subdivisions: (1) streams in which the basin is eroding and the bottom, therefore, is generally firm; and (2) streams in which material is being deposited and, therefore, the bottom is generally composed of soft sediments. In many cases these situations alternate in the same stream, as may be seen in the ‘’rapids’’ and ‘’pools’’ of small streams. Aquatic communities are quite different in the two situations owing to the rather different conditions of existence. The communities of pools resemble those of ponds in that a considerable development of phytoplankton may occur and the species of fish and aquatic insects are the same or similar to those found in ponds and lakes. The life of the hard bottom rapids, however, is composed of more unique and specialized forms, such as the ner spinning caddies (larvae of insects called caddies flies or trichoptera), which constructs a fine silk net that removes food particles from the flowing waters.

        The load of sediment discharged into the oceans by the great rivers of the world tell us something about man’s treatment of the land. The rivers of Asia, the continent with the oldest civilizations and the most intense human pressure on the land, discharge 1500 tons of soil per square mile of land area annually.