180 



THE WORK OF RUNNING WATER 



Here it is no longer cutting down its trough, but has 

 only sufficient slope to enable it to bear along its load of 

 waste. It here deposits upon its valley floor about as 

 much as it takes away. In this part of its course a river 

 is said to be graded. The longer a river flows undisturbed 

 by any deformation of its valley, the fewer falls and rapids 

 it will leave and the longer will be its graded stretches. 

 The Missouri River near Marshall, Missouri, is an excellent 

 example of a graded river. 



Sometimes a stream becomes so overloaded with detritus, 

 which it has acquired in a steeper part of its extent, or 



PLATTE RIVER 



which has been brought to it by tributaries, that it is con- 

 tinually being forced to deposit some of its load. Thus it 

 silts up its course and flows in a network of interlacing 

 shallow channels. The Platte as it crosses the plains of 

 Nebraska is an example of such an overloaded river. 



When a stream swings around a curve, the swiftest part 

 of the current is on the outside of the curve and the slowest 

 on the inside. A river that is carrying about all the load 

 that it can, on passing around a curve, is able in its outer 

 part to carry more than before and cuts into the bank, 

 while on its inner part it flows less rapidly and is able to 



