io8 



WORK OF RUNNING WATER 



therefore look to the places where these changes in velocity occur 

 for the principal deposits of running water. 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. 

 Most streams increase in size as they flow, but to this general rule 

 there are exceptions, (i) If a stream flows through a very dry re- 

 gion it receives few tributaries, while evaporation is great and the 



thirsty soil and rock 

 through which it flows 

 absorb some of its water. 

 In such a region a stream 

 may diminish as it flows, 

 and may even disappear 

 altogether (Pis. II and X). 

 (2) In some places certain 

 streams break up into 

 several (Fig. 107), and in 

 this case the volume and 

 therefore the velocity of 

 each is less than that of 

 the original stream. (3) 

 Many streams, especially 



Fig. 107. Delta of Lake St. Clair. 

 Survey Chart.) 



(Lake in arid regions, have much 

 of their water withdrawn 

 for irrigation. (4) During the decline of their floods, all streams 

 decrease in volume and velocity. 



Location and Forms of Alluvial Deposits 



i. At bases of steep slopes. The water of every shower 

 washes sediment down the slopes of hills, and much of it is left at 

 their bases. Its lodgment there, even where there are no valleys 

 or gullies, is shown in some places by the burial of fences by the mud 

 lodged against them. Temporary streams flowing down steep 

 slopes are checked suddenly at their bases, and abandon there their 

 heavy loads of debris. Thus, at the lower end of the new-made 

 gully on the hillside there is commonly a mass of detritus which 

 was washed out of the gully itself (Figs. 40 and 108). Material in 

 such positions accumulates in the form of a partial cone, known as 

 an alluvial cone. Alluvial cones have much in common with cones 



