Rivers and Valleys 



river is continually struggling with this detritus. 

 It deals with this burden in the following man- 

 ner: The motion of the stream is swiftest in 

 its central parta, because, in most cases, the 

 water is deepest in that part of its bed, and is 

 therefore the least influenced by friction. On 

 the sides of the stream where the water is shoal, 

 the current is least swift; therefore in these mar- 

 ginal parts it constantly tends to lay down sedi- 

 ments. As soon as the alluvial terrace is formed, 

 certain kinds of trees, particularly our willows 

 and aspens, find a lodgment upon it. They 

 push their roots out into the nutritious mud 

 and enmesh it in their net-work of fibres; they 

 also send up from these roots a thick hedge of 

 stems, in which the flood- waters lose their swift- 

 ness of motion and therefore drop their con- 

 tained sediments. In the state of nature, all our 

 American streams, and those of most other 

 countries as well, are bordered by a close array 

 of these plants, all of which are at work to win 

 against the channel of the stream. But for the 

 cutting power of the stream, they would quickly 

 close its channel; as it is, they constantly crowd 

 its waters within a narrow pathway. 



Against the encroachments of the alluvial banks 

 brought about by the action of the' water-loving 

 trees, the river prevails by fits and starts, under 

 the action of a curious law which causes its current 

 to rebound from bank to bank. The nature of this 

 principle of rebounding can best be seen by ob- 

 serving the effect arising where a jetty is built at 

 147 



