EROSION AND SEDIMENTATION 



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quite a time, and then it is small and lasts for a much longer 

 time than that in the open field, and when the stream stops 

 flowing, the soil of the woods is still very moist (fig. 105). 

 The trees and undergrowth retard the falling rain. Humus 

 acts as a sponge in holding large quantities of water, so that 

 streams start slowly, and when they do start, the leaves, twigs, 



FIG. 106. Early stage in erosion of deforested field 



Deep gullies have been washed in the hillside, but they do not extend into the 

 adjacent forested area. Photograph by United States Forest Service 



bushes, etc. retard the movement so that the ability of the 

 water to remove soil and to carry it is held in check. The 

 humus retains much of the moisture after the stream has 

 stopped flowing. Furthermore, the covering of trees etc. tends 

 to reduce the loss of water by evaporation. Compare the 

 woods with the open fields and apply the facts to the question 

 of erosion. 



