RIVER DEVELOPMENT 339 



the river will slope away from the channel instead of 

 toward it. 



This is well shown in the lower Mississippi, where the 

 river is found to be flowing on a natural embankment, the 

 side streams running away from the river instead of into 

 it. In some places the embankment is fifteen or twenty 

 feet above the rest of the flood plain. These natural 

 levees, as they are called, often force the tributary streams 

 to flow for long distances upon the flood plain before they 



LEVEE OF THE SACRAMENTO. 



can enter the main river. The Yazoo River is forced to 

 flow along the flood plain some 200 miles before it can 

 enter the Mississippi. Artificial levees are often built to 

 keep rivers from overflowing their flood plains. Such are 

 the high levees along the Lower Mississippi and Sacra- 

 mento rivers. 



Sometimes the flood plain of the main river is built up 

 more rapidly than the tributaries can build theirs, so that 

 they are dammed up as they enter the flood plain of the 

 main stream and form a series of fringing lakes along its 

 border. A fine example of this is found in the lower 

 course of the Red River of Louisiana. 



