196 GEOLOGY. 



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. Even 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 veloci- 

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

 one time and fine at another. A similar sequence of deposits takes 

 place in the flood-plain of a meandering stream, irrespective of 

 floods. Flood-plain deposits are often therefore very heterogeneous, 

 as shown in Fig. 186, which represents the constitution of the alluvium 



Omaha Section 



bfue> 



V^^" 



Fig. 186. — Diagram to show the heterogeneous character of alluvial deposits. 

 (Todd, Bull. 158, U. S. Geol. Surv.) 



of the Missouri River at Omaha. The deposits of the streams range 

 from the finest clay, through sand to gravel, and even bowlders. In 

 general they become finer down-stream. In a given plain, they are 

 usually coarser below and finer above. 



Topography of the flood-plain. — The flood-plain is nearly, but 

 not altogether, flat. It has a gentle slope down-stream, and often for 

 a distance from the sides toward the center (Fig. 174). This latter 

 slope is the result of deposition by waters descending to the plain from 

 the sides. It is destroyed wherever a meandering stream reaches its 

 bluffs. When levees are well developed, there is a slope from them 



