WORK OF RUNNING WATER 



one time and fine at another. Many flood-plain deposits are, 

 therefore, very heterogeneous, ranging from the finest mud, through 

 sand, to gravel, and even bowlders. In general they become finer 

 down-stream. 



Flood-plain meanders. A stream with an alluvial plain is 

 likely to meander widely (Pis. XI and VII). In general terms 

 this may be said to be the result of low velocity, which allows 

 the stream to be turned aside easily. Were the course of such a 

 stream made straight, it would soon become crooked again. The 

 manner of change is illustrated by Figs. 115 and 116. If the 

 banks are less resistant at some points than at others, as is always 

 the case, the stream will cut in at those points. If the configuration 

 of the channel is such as to direct a current against a given point, 



b (Fig. 115), the result is the 

 same, even without inequality 

 of material. Once a curve in 

 the bank is started, it is in- 

 creased by the current which 

 is directed into it. Further- 

 more, as the current issues from 

 the curve, it impinges against 

 the opposite bank and develops 

 a curve at that point. The 

 water issuing from this curve 

 develops another, and so on. 



Once started, the curves or 

 meanders tend to become more 

 and more pronounced (Fig. 

 116). In the case represented 

 by Fig. i, PI. VII, the narrow 

 neck of land between curves is 

 almost cut through. A later 

 stage in the process is shown in 

 Fig. 2. When the stream has 

 cut off a meander, the aban- 

 doned part of the channel may 

 Fig. 117. Meanders and cut-offs in the remain unfilled with sediment. 

 Mississippi Valley below Vicksburg. The If it contains standing water, 



(Fig. 117). Some such lakes 



