78 WORK OF RUNNING WATER 



toward the center, and the varying velocities of the different parts 

 of a stream serve a similar purpose. A river is therefore to be 

 ~ looked upon as a multi- 

 tude of currents, some 

 rising from the bottom 

 toward the top, some 

 descending from top to 



Fig. 67. Diagram to illustrate the effect of bottom, some diverging 

 irregularities, a and b, in a stream's bed, on the from the center to- 

 current striking them. ward the s j des and 



some converging from the sides toward the center. The sum of 

 the upward currents is of course always less than the sum of the 

 downward, so that the aggregate motion of the water is down slope. 



Sediment in suspension is held up chiefly by the upward currents, 

 which, locally and temporarily, overcome the effect of gravity. 

 The particles in suspension are constantly tending to fall, and fre- 

 quently falling; but before they reach the bottom, many of -them are 

 carried up by subordinate currents, only to sink and be carried up 

 again. Even if they reach the bottom, as they do frequently, they 

 may be picked up again. It is probable that every particle of sedi- 

 ment of such size that it would sink readily in still water is dropped 

 and picked up many times in the course of any long river journey, 

 and its periods of rest often exceed its periods of movement. 



Corrasion. The mechanical wear effected by running water is 

 corrasion. So long as the materials to be moved are incoherent, it 

 is easy to understand how running water moves them. The water 

 which flows over the surface of a cultivated field gathers earthy mat- 

 ter, and the process is continued all the way to the channel of the 

 stream. Thus sediment is gathered at the very sources of flow, 

 and the stream gathers load from its bed wherever it flows with 

 sufficient velocity over loose material. Streams also undercut 

 their banks, and receive new load from the fall of the overhanging 

 material. 



The larger part of the sediment of streams is made up of mate- 

 rial loosened in advance by weathering; but many rivers wear rock 

 which is not weathered, for the principal valleys of the earth are in 

 solid rock, and many of them in rock of great hardness. How does 

 the stream wear the solid rock? 



When a stream flows over a rock bed, the wear which it accom- 

 plishes depends chiefly on the character of the rock, the velocity 



