LAND WATERS STREAMS 



183 



swept away in time of flood, but they sometimes become more or 

 less permanent islands. 



The profiles of most valleys are curves, the curvature becoming 

 less as the lower end of the stream is approached (Fig. 151). It 



SEA LEVEL 



Fig. 151. Profile of a normal valley. 



therefore happens that as a stream descends its valley it generally 

 reaches a point where its reduced gradient so diminishes its velocity 

 that it must abandon some of its load. In this way sediment is 

 distributed for long distances along valley bottoms. It is left in 



\ 



A 





Fig. 152. Flat developed by aggradation diagrammatic. 



the channels of streams in low water, and spread over their flood 

 plains in high water, aggrading them and making them alluvial 

 plains. Deposition in a valley which has no flat tends to develop 

 one (Fig. 152). Alluvial deposits on valley flats are usually but 



Fig. 153. Levees of the Mississippi in cross-section, four miles north of 

 Donaldsonville, La. Vertical scale x 50. The horizontal line represents 

 sea-level. The bottom of the channel is far below sea-level at this point. 



a few feet, or at most a few scores of feet thick; but in some cases 

 they reach hundreds of feet. 



Natural levees (Fig. 153) are developed in flood plains aggraded 

 by occasional floods. At such times the current in the main chan- 

 nel is swift; but as the water escapes its channel and spreads over 



