STREAMS AND THEIR WORK 



179 



swift ; third, its flood plain has been formed by lateral 

 erosion ; fourth, its course is rather crooked, and winds 

 back and forth across the flood plain ; fifth, its valley 

 is wide and its sides are not steep, the V-shaped val- 



Photoyraph by Detroit Publishing Company. 

 FIG. 70. A river in maturity. 



ley of the young stream having disappeared by erosion ; 

 sixth, waterfalls are few, for most of them have 

 been destroyed by erosion ; seventh, unless the lakes 

 were very large, erosion and sedimentation have caused 

 them to disappear ; eighth, a mature stream carries a 

 moderate amount of sediment. The Middle Mississippi, 

 the Ohio, and the Wabash Rivers show the result of 

 these conditions. 



Old age. In an old stream we find the influence of 

 erosion still more marked: 



