Chaptee 22 



EFFECTS OF SOIL CONSERVATION 

 Cael B. Beown 



Sedimentation Specialist 



Office oj Research 



Soil Conservation Service 



Washington, D. C. 



CAUSES AND RATES OF SEDIMENT PRODUCTION 



This chapter is concerned with erosion and sediment production on 

 watersheds: their lands, drainways, and stream systems; with the rates 

 and sources of sediment production ; with the effects of erosion and sedi- 

 mentation on man's use of land and water resources; and with the 

 potentialities for soil and water conservation. 



Geologically normal rates of erosion and sedimentation materially 

 affect man's enterprises in some watersheds, mainly in arid regions. 

 By far the largest number of sediment problems stem, however, from 

 man's disturbance of the prehistoric geologic norm (Lowdermilk, 1934) . 

 Clean-cutting and burning of forest and brush lands, overgrazing of 

 grasslands, and the cultivation of crop lands have greatly reduced the 

 primeval vegetal protection of the land surface against the erosional 

 attack of raindrops, surface runoff, and wind action (Bennett, 1939; 

 Glenn, 1911). The resulting erosion of the soil mantle over much of 

 the earth has been accelerated far beyond the rate of soil formation, 

 with which it was formerly in balance on most sloping lands. The 

 associated formation of countless gullies and valley trenches has greatly 

 increased channel density (length of channels per unit area). The 

 erosion of these new channels directly contributes large quantities of 

 sediment to stream loads (McLaughlin, 1947; Thornthwaite et al., 

 1942) . They also act as flumes through which erosional debris can be 

 delivered more rapidly from much larger areas of the land surface to 

 geologically developed stream systems. This, alone, has changed the 

 regimen of many streams, setting up a chain of events that materially 

 affects the utilization of such natural resources as stream flow, reservoir 



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