ALLUVIAL DEPOSITS 



ically t<> a notable extent, and the deepening of the channel takes 

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

 other words, the stream in flood aggrades its plain, and degrades 

 it-> i hannel. This follows from the fact that the current is slow 

 on the plain, where the water is shallow, and rapid in the channel, 

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

 while the current was torrential, is filled up again by sediment from 



Fig. 115 Fig. 116 



V'\K. 115. Diagram illustrating an early stage in the development of river 

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

 Fig. 116. A later stage in the development of meanders. 



the feebler current. This alternate deepening and filling is scour- 

 and-jlll. It is well illustrated by the Missouri River. At Nebraska 

 City, scour reaches depths of 70 to 90 feet occasionally. At Blair, 

 about 25 miles above Omaha, the same river is believed to cut to 

 bed-rock (about 40 feet below the bottom of the channel in low 

 water) during floods. All streams similarly situated do a like work. 

 The material thus eroded is shifted down-stream, some of it for short 

 distances only, and some of it to the sea. An aggrading stream, 

 therefore, is not without erosive activity; it is a stream whose fill 

 exceeds its scour, not one which has ceased to erode. 



Materials of the flood-plain. As a result of its varying velocities 

 in flood and low water, a stream may deposit coarse material at 



