184 



GEOLOGY 



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

 depth suddenly becomes less. It must therefore abandon 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. Aggrading streams deepen their channels period- 

 ically to a notable extent, and the deepening of the channel takes 



Fig. 154 Fig. 155 



Fig. 154. Diagram illustrating an early stage in the development of river 

 meanders. The dotted area represents the area over which the stream 

 has worked. 



Fig. 155. A later stage in the development of meanders. 



place at the very time when the flood-plain is being aggraded. In 

 other words, the stream in flood aggrades its plain, and degrade- 

 its channel. This follows from the fact that the current is slmv 

 in the former position, where the water is shallow, and rapid in 

 the latter, where it is deep. After the flood subsides, the channel. 

 deepened while the current was torrential, is filled up again by sedi- 

 ment from the feebler current. This alternate deepening and filling 

 is known as scour-and-fill. It is well illustrated by th< Missouri 

 River. At Nebraska City, scour reaches depths of 70 to 90 feet 



