Rivers and Valleys 



.As soon as the stream is more than two or three 

 feet wide and a foot in depth, we begin to see 

 evidences of its energy. Even if the fall be but 

 at the rate of fifty feet to the mile, we shall find 

 that such a stream is able to urge forward with 

 great violence masses of stone several inches in 

 diameter. If we roll a stone the size of a man's 

 head into the channel, it is swept along, bumping 

 violently against the obstacles it encounters, 

 striking first one rock-bank and then another, 

 until it becomes fixed in some crevice. If, after 

 the pebble has journeyed for a few hundred feet, 

 we recover it from the stream, it is often easy 

 to note the dents on its surface, produced by the 

 collisions on its journey. In most cases there 

 has been a corresponding blow and an equal 

 wearing inflicted on the firm rocks against which 

 it collided. 



A little observation with streams having dif- 

 ferent rates of fall will show the observer that 

 the ease with which a stone is urged onward, and 

 the size of those which a stream of given volume 

 can carry, depend in a remarkable way on the 

 rate of its descent toward the sea level, and there- 

 fore on the velocity with which its waters flow. 

 Computation and experience have shown that 

 this increase in speed is proportionate at least to 

 the cube, or third power, of the velocity with 

 which the current flows. One distinguished 

 student of this hydraulic problem has come to 

 the conclusion that the increase of the propulsive 

 power of the stream upon the fragments which 

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