178 



GEOLOGY 



have already seen that gradient and volume are the most important 

 factors in determining the velocity of a small stream. 



1. Loss of velocity. The commonest cause of loss of velocity 

 is decrease of gradient. Running water may lose velocity (1) 

 suddenly, as when it passes from a steep slope, whether of hill or 

 mountain, to a gentle one, or to a body of standing water; or (2) 

 slowly, as in descending a valley the gradient of which becomes 

 gradually less. We therefore look to the places where these changes 



velocity occur for the principal deposits of running water. 



m 



HHBIHBBHHHBHH 



Fig. 146. Delta of Lake St. Clair. (Lake Survey Chart.) 



Streams also become slower wherever their channels become wider, 

 even if volumes and gradients remain constant. 



Decrease of volume is a less common cause of decrease of velocity. 

 Streams generally increase in size with increasing distance from 

 their sources, but to this general rule there are exceptions. (1) 

 If a stream flows through a very dry region, it may receive few 

 tributaries and few springs, while evaporation is great and the thirst y 

 soil and rock through which it flows absorb some of the water. 1 1 1 

 a dry region, therefore, a stream may diminish as it flows, and niny 

 even disappear altogether (Pis. II and XIII). (2) A stream some- 

 times breaks up into several streams (Fig. 146). The volume of 



