SURFACE WATER, DRAINAGE, AND IRRIGATION 305 



As the softer portions of the rocks are dissolved by the 

 water, they break apart ,1m til the pieces are small enough 

 to be moved by the running water. The larger pieces are 

 usually started in time of high water, since the carrying 

 power of running water increases enormously as the veloc- 

 ity is increased. As the loose material is rolled along, the 



FIG. 260. An Example of Water Erosion. 



solid surfaces in the bed of the stream are cut down and 

 the material produced is moved away by the force of the 

 current. This work is called water erosion. 



Erosion is taking place over nearly all of the exposed 

 surface of the earth. When there is a heavy rain, many 

 miniature streams are formed on the surface of the earth. 

 These little streams all do their share in erosion, each one 

 carrying away a small part of the surface material and each 



