112 



WORK OF RUNNING WATER 



a new course over the valley flat. This process may be repeated 

 again and again (Fig. in). Some streams deposit bars in their 

 channels, especially in low water. The bars may be swept away 

 in time of flood, but some of them become more or less permanent 

 islands. 



The profiles of the bottoms of most valleys are curves, the curva- 

 ture becoming less as the lower end of the stream is approached 



Fig. 112. Profile of a normal valley, showing decreasing slope down stream. 



(Fig. 112). It 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 bot- 

 toms. It is left in 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. 113). Alluvial deposits 



X 



i 



Fig. 113. Flat developed by aggrada- on valley flats are usually but 

 tion diagrammatic. a few feet, or at most a few 



scores of feet thick; but in rare cases they reach hundreds of feet. 



Natural levees (Fig. 1 14) are developed on 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 



Fig. 114. Levees of the Mississippi in cross-section, four miles north of Donald- 

 sonville, La. Vertical scale X 50. The horizontal line represents sea-level. The 

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



the adjacent flat, its velocity is checked promptly, because its 



depth suddenly becomes less. It therefore abandons much of its 



load then and there. Repeated deposition in this position, in excess 



of that over other parts of the flood plain, gives rise to the levees. 



Scour-and-fill. 1 Aggrading streams deepen their channels period- 



1 Hill, Erosion and Deposition by the Indus. Geol. Mag., July, 1910. 



