EROSION AND SEDIMENTATION 



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fine material may not be dropped for a long time. If a 

 tumbler of muddy water is allowed to stand, an estimate may 

 be made of the time required for all the silt to settle to the 

 bottom of the tumbler. This will show how long the water, 

 if moving, might have carried silt. 



If one digs up two squares of soil of similar size one 



bare and the other of turf and both are then subjected to a 



FIG. 146. Erosion by a stream, Tumwater Cafion, Washington 



A swiftly flowing stream wears down its bed rapidly and forms a steep-walled 

 valley. Photograph by W. B. MacCallum 



stream of water from a hose, it will be seen that the turf 

 withstands the running water much longer than does the bare 

 soil. Erosion takes place less rapidly when soil is covered or 

 when it is held by plant roots. 



In wooded regions, where there is much undergrowth and 

 humus, the water from rain flows from the surface very slowly. 

 In an open field the same rain will produce quite a stream. 

 The stream upon the field is soon gone, and the surface 



