.millkk] 



RUNNING WATER 



81 



Figure 1. — Terraces along the side of a small ravine. 



of deposition has become apparent to the student. He has noticed 

 deposits at the mouth of the gully, on the inside of the meanders, 

 and at the mouth of the stream where it flowed into quiet wat^r. 

 \Miat causes a stream to drop its load? is evidently the key to 

 an explanation of these deposits and is the problem that he must 

 solve first. He observes that the current goes slower on the in- 

 side of the bend than on the outside, that it is checked when 

 it flows into a body of quiet water or when it flows from a gully 

 onto the more gentle gradient at the base of the slope. In other 

 words he has learned that, a stream drops sediment zvhen its 

 current is checked, and some of the ways by which the current 

 may be checked. From these facts he can reason out other 

 causes if field examples are not available. For example, such 

 causes as : gradient of the valley becomes less toward the mouth . 

 loss of volume due to sinking into the ground, to evaporation, 

 to withdrawal for irrigation, to the dividing of the stream into 

 many channels ; and overload. 



Places of deposition and types of deposit. — That there are 

 three chief places of deposit by running water, viz., at the base 

 of slopes, on valley bottoms, and at the mouths of streams, be- 



