364 THE WORK OF RUNNING WATER 



the rocks are carried in solution. Gravel, pebbles, and big 

 stones are too heavy for the stream to lift, but they are rolled 

 and dragged along (Fig. 230). As the river sweeps onward, 

 the gravel, stones, and bowlders which it carries rub and grind 

 against the soft banks and bottom and set free new particles 

 that are carried away in suspension, or they hit against rocky 

 banks and bottom and chip off small and large pieces of rock. 

 These new rock pieces are in turn rolled and dragged along 



by the stream and 



serve as additional 

 tools in its work of 

 tearing, cutting, and 

 grinding. As a result 

 of the grinding and 

 rubbing, the stream 

 bed deepens and 

 the banks wear away 

 or erode (Fig. 231). 

 Rivers whose beds 

 and banks are of 

 rocky material too 



FIG. 231. -A stream wearing away its bank. heayy tQ be bome 



away do not deepen their channels and wear away their banks 

 as rapidly as rivers whose beds and banks are made up of 

 sandy or loose soil. The Tennessee River is eroding its banks 

 and is giving great anxiety to the farmers who own land along 

 its course. 



Streams build up land. Rain flowing down a gully carries 

 away soil but drops most of it at the base of the gully (Fig. 

 229). Mountain streams deposit great masses of debris at 

 the foot of the mountain where they change from steep 

 swift-flowing mountain streams to gentler-sloping, quieter 

 valley streams. The load that a stream can carry depends 



