SURFACE WATER, DRAINAGE, AND IRRIGATION 309 



FIG. 265. The Mississippi River Delta. 



As the current of a river decreases in velocity, its carry- 

 ing power is greatly lessened, and some of its load must be 

 deposited. The Mississippi River flows very slowly in 

 the lower part of its course 

 and as a result has been 

 filling up its channel. 

 There are many bars in 

 the lower Mississippi which 

 interfere with navigation. 

 Boats require an experi- 

 enced pilot because of 

 these shifting bars. As 

 the bed of a river has 

 been raised it has been necessary to build high artificial 

 banks (levees) to keep the water from overflowing the 

 agricultural land and doing great damage. The Govern- 

 ment of the United 

 States annually 

 spends millions of 

 dollars on these 

 levees. 



The sediment 

 left by streams on 

 the land in times of 

 overflow is called 

 alluvial deposit. 

 The more the cur- 

 rent is checked, the 

 greater will be the 

 deposit. In some 

 places where the current is not checked the swiftly moving 

 water may take up soil instead of depositing it. Where 

 silt is deposited, the land will be enriched ; while of course 



FIG. 266. A River Delta. 



