THE WORK OF STREAMS 



159 



while its tributary valleys were not, and the ice were to 

 disappear subsequently, the bottom of the main valley would 

 be lower than the mouths of the tributaries, whose streams 

 would descend by rapids or falls. Rapids and falls of this 

 origin are common in some of the mountains of western 

 United States (p. 222). Such falls are consequent upon de- 



FIQ. 155. Twin Falls of Snake River, Idaho. (U.S. Geol. Sun.) 



clivities which the rivers had no share in forming, and so 

 have been called consequent falls. 



A second class of falls is due to escarpments which the 

 streams helped to make ; these are called subsequent falls. 

 A river flowing on the high gradient shown in Figure 156 is 

 likely to be a degrading river. Obviously, it will wear its 

 channel faster at A, where the rocks are soft, than just 

 above, where they are hard. The result is that the gradient, 

 and hence the velocity, of the stream becomes greater at A 

 than elsewhere (Fig. 157). In other words, rapids are formed, 

 which become increasingly swift as the gradient becomes 



