SURFACE WATER, DRAINAGE, AND IRRIGATION 303 



wears away its bed and thus lowers the level of the water 

 behind it. Lakes may J)e considered as enlargements in 

 a river's course. The river is constantly at work to remove 

 this enlargement both by filling and by draining the lake. 

 There are no lakes in an old river system for this reason. 

 Lakes are quite useful to civilized man. They often 

 form valuable inland waterways, as in the case of the 



FIG. 259. Flood, Dayton, Ohio (March, 1913). 



Great Lakes of North America, the Caspian Sea, or the 

 Lakes of Africa. Being quite large they act as reservoirs 

 for excess water. There are no floods in the St. Lawrence 

 River system, while in the Mississippi valley there are 

 destructive floods almost every year (Figure 259). 



When the rainfall of a region is insufficient to furnish 

 enough water to fill depressions to overflowing, salt 

 lakes occur. The streams which flow into the depression 



