26 THE PRINCIPLES OF SOIL MANAGEMENT 



is exemplified in every stream of whatever size. Where 

 the flow is checked and thereby the carrying power 

 reduced, some of the coarsest material is deposited. 

 Where the flow is increased, instead of deposition, 

 coarser material is picked up. Changing an obstruction 

 causes extensive regrading of the channel by the current. 

 Bends in the stream which require a greater velocity on 

 one side of the channel than on the other cause the same 

 sort of rearrangement, and this is nicely illustrated 

 in the meandering of streams. They wind over their 

 course always cutting away the material on the outer 

 side of the curves, and depositing it on the inner side 

 of the curves lower down. Thus the stream is continually 

 changing its course. It meanders from one side of its 

 flood-plane to the other. It cuts off large curves and 

 proceeds to form new ones. All these processes may 

 be observed in any rivulet, yet they are the exact 

 counterpart of the things which are taking place in 

 every large river valley. Careful determinations re- 

 ported by Bobb, show that the large rivers of the world 

 remove annually in suspension the following amounts 

 of material: 



Height in feet of Thickness of sedi- 



column of sedi- ment, in inches 



ment with base if spread over 



1 mile square. drainage area. 



Mississippi river 241.4 .00223 



Potomac 4.0 .00433 



Rio Grande 2.8 .00116 



Uruguay 10.6 .00085 



Rhone 31.1 .1075 



Po 59.0 .01139 



Danube 93.2 .00354 



Nile 38.8 .00042 



Mean 76.65 .00614 



