180 



GEOLOGY. 



The third way by which velocity is decreased as the result of de- 

 creasing volume is illustrated at the debouchures of many streams. 

 Near the Gulf, for example, the Mississippi branches repeatedly (see 

 Fig. 190). The same phenomena are often seen where one stream 

 joins another (Fig. 169). Individually the distributaries are much 

 smaller than the main stream before they separated from it, and be- 

 cause they are smaller their combined surfaces are greater, and the 

 amount of energy consumed in the friction of flow is increased. The 

 velocity of the water and its carrying power are, therefore, reduced. 

 Thus the branching of streams gives rise to deposition, and where 

 deposition takes place the gradient of the stream is reduced, and this 



Fig. 169. — Delta of the Chelan River at its junction with the Columbia. Shows the 

 tendency of streams to distribute where active deposition is in progress. (Willis, 

 U. S. GeoL Surv.) 



occasions still further deposition. The sediment which fills up the 

 channel and checks the flow finally compels the stream, or some part 

 of it, to transgress its banks. Deposition, therefore, favors the de- 

 velopment of distributaries, and the development of distributaries in 

 turn favors deposition. 



The foregoing statements make it clear that a stream may be erod- 

 ihg in one part of its valley while it is depositing in another, and that 

 drofeion may alternate with deposition in the same place, on account 

 erf fluctuations in volume, and, therefore, in velocity of the stream. 

 It will be seen in the sequel that erosion and deposition may be taking 

 place at the same time in the same part of the valley. The activities 



