THE WORK OF RUNNING WATER 59 



not only prevented further deposition of sediment there, but forced 

 the river to clear out its own channel. This change permitted 

 ocean vessels to reach New Orleans, and insured the commercial 

 prosperity of that city. 



Erosion defined. The wearing away of the land surface is 

 erosion. In general, erosion consists of three more or less distinct 

 processes. These are (1) the loosening of the rock, often by weath- 

 ering, (2) the picking up of the loosened material, and (3) its trans- 

 portation. When the running water is no longer able to carry away 

 sediment, it ceases to erode its bed, except by solution. 



Deposition a result of erosion. When the velocity of a stream 

 is checked, it generally drops some of the sediment it was carrying. 

 This is always the case if it had as much sediment as it could carry 

 before its velocity became less. Some of the sediment is left in the 

 valleys, especially toward their lower ends, and some of it is carried 

 to the sea, or to the lake or other basin to which the river flows. 

 Deposits of sediment in valleys build up (aggrade) their bottoms. 

 Thus the Mississippi is spreading sediment over the bottom of its 

 valley for hundreds of miles north of the Gulf of Mexico, and many 

 other large streams are doing the same thing. The amount of sedi- 

 ment deposited on low land by running water is, however, far less 

 than the amount worn away from the high land. 



Changes Made by Rivers in their Valleys 



A valley has three dimensions, depth, width, and length, and 

 each dimension is subject to change. 



The deepening of valleys. Swift streams make their valleys 

 deeper, but many slow streams deposit more sediment than they 

 take away, and so make their valleys shallower. Many streams 

 deepen their valleys in their upper courses where their waters are 

 swift, while they make them shallower in their lower courses where 

 the currents are sluggish. 



The principal reason why a stream is swift is that its channel 

 has a steep slope; but as such a stream deepens its valley, the slope 

 or gradient of its channel becomes less, and the stream flows more 

 and more slowly. In time, every swift stream will cut its channel 

 down until its current becomes sluggish. 



