THE WORK OF STREAMS 



177 



FIG. 184. Diagram of a flood plain 

 formed by deposition in a narrow valley. 



What is the age of the rock valley in 

 which the filling has occurred, and how is 

 it shown ? What work was the river do- 

 ing before filling commenced ? The evi- 

 dence ? What things may have forced 

 the river to cease its earlier work and ag- 

 grade its valley ? 



Normal flood plains are widest in their lower portion, 

 where the gentle gradient favorable to lateral shifting was 

 developed first, and be- 

 come narrower more or 

 less regularly up valley. 

 The lower Mississippi has 

 opened a flood plain from 

 20 to 60 miles or more 

 wide. The downstream 

 slope of flood plains varies 

 with the volume of the 

 stream and the character 

 of the material it deposits. 

 Relatively small streams 

 heavily overloaded with 

 coarse material build 

 steep flood plains, sometimes with a descent of 50 to 75 

 feet a mile; large rivers, depositing fine sediment, build 

 nearly level flats. 



Natural levees. During times of flood a river deposits 

 most actively along the edges of the channel. Here the 

 depth of the overflowing water is diminished suddenly and, 

 in consequence, its velocity and carrying power. Here 

 during the continuance of the overflow the marginal waters 

 of the main current are checked by friction with the slower 

 moving backwaters. Deposition along these lines during 

 many overflows may build low, marginal ridges with a 

 gentle slope away from the river. Such embankments are 



natural levees (Fig. 

 185). It is evident 

 that natural levees 

 will not prevent 

 subsequent over- 

 flow, since a stream 

 , can build them only 



rio. 185. Diagram showing natural levees and . 



the general structure of stream-laid beds. to the level of its 



