THE WORK OF STREAMS 137 



the problem, as, for example, the fact that streams cannot re- 

 duce their basins quite to sea level (p. 139) and that an average 

 of 2300 feet would not have to be eroded away to bring the 

 land to the level of the sea, for the surface of the ocean would 

 be raised by the deposition in it of the waste from the land. 

 Nevertheless, such computations are worth while, since they 

 aid one to appreciate the importance of the work being done 

 by running water. 



Questions 



1. Why is the Niagara River practically free from sediment? 



2. Other things being equal, would a given stream corrade faster 

 when flowing across the edges of highly tilted beds, or on horizontal 

 beds ? Why ? When the beds dip downstream or upstream ? Why ? 



3. Is corrasion favored more by a constant volume, or by great 

 and sudden fluctuations in a stream ? 



4. Enumerate all the conditions which might enable one of two 

 streams of equal and constant volume to corrade much faster than 

 the other. 



5. Is it possible for a stream to corrade without degrading ? To 

 degrade without corrading ? 



6. Will a given stream flow faster when fully loaded with coarse 

 or with fine material ? 



Most streams flow in valleys. In general, valleys correspond 

 in size to their streams, and, like the stream it contains, a 

 given valley is smaller than the one it joins, and larger than 

 those which join it. At their union, the bottoms of tributary 

 valleys are normally at the same level as the bottoms of the 

 larger valleys to which they lead. Furthermore, all streams 

 are engaged, with the help of the agents of weathering, in en- 

 larging their valleys. These facts indicate that the valleys 

 were not found ready-made by the streams which occupy them, 

 but that they are a result of the work of the streams aided by 

 weathering agents. Many synclinal troughs (p. 66) form 

 valleylike depressions. Since they are due to the structure 



