Local Habitats, 



Communities, and 



Succession: 



Streams 



When rainwater falls on an uneven surface, it col- 

 lects in depressions. As the water overflows them, 

 the current erodes a narrow channel that deepens 

 with each succeeding shower and may eventually 

 drain the depression. There is usually also a lateral 

 meandering of the stream, by which a valley is 

 formed. The site of the headwaters of such streams 

 is impermanent, and continued erosion forces the 

 headwaters and the channel farther and farther back 

 into the upland. The stream is at first a temporary 

 one, dependent for its waterflow on rainfall runoff, 

 but when its channel is cut below the level of the 

 groundwater table the stream becomes permanent, 

 fed by general, continuous seepage. The headwaters 

 of such a stream are therefore its youngest portions 

 physiographically, and the stream is progressively 

 more aged towards its mouth. 



In hilly or mountainous terrain, water may ac- 

 cumulate in large basins until ponds or lakes are 

 formed. In the Great Basin of North America, such 

 lakes have not found an outlet to the sea, and evapo- 

 ration has left them with a very high salt content. 

 Ordinarily, however, the water level in such a lake 

 will rise until it overflows at the lowest point on the 

 perimeter. Then the waters continue to flow down- 

 ward until they eventually reach the sea. Streams 

 springing from fixed headwaters (melting snowfields 

 and glaciers, springs) carve valleys that are of essen- 

 tially the same age throughout. Streams less than 

 3 m (10 ft) wide are usually called creeks or brooks; 

 rivers are streams 3 m or more wide. 



A river system in youth is characterized by val- 

 leys that are narrow and steepsided ; the flow of water 

 is usually fast, there are few tributaries, and there 

 are many waterfalls, ponds, and lakes along its 

 course. As the river system matures, its valleys be- 

 come wider, its slopes more gentle, and its tribu- 

 taries more numerous and longer. Many ponds and 

 lakes are drained, and waterfalls are worn down to 

 rapids or riflles. The areas of upland are well dis- 

 sected, and the land is thoroughly drained. In old 

 age. the river system has reached base-level. The 

 upland has been worn down to low ridges between 

 tributary river valleys, and the region as a whole is 

 called a peneplain. There are no lakes, ponds, or 

 rapids, and the flow of water is sluggish (Strahler 

 1951). 



HABITATS 



Exclusive of its lakes, the principal habi- 

 tats in a stream are falls, rapids or rijfles. sand- 

 bottom pools, and mud-bottom ponds. The character 

 of the bottom depends primarily on the velocity of 

 the water current, which, along with the volume of 

 stream flow, can be readily measured (Robins and 



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