Ch. 22] POTENTIALITIES FOR SEDIMENT CONTROL 401 



sical considerations, therefore, primarily govern the amount of re- 

 duction in sediment production that can be achieved in many water- 

 sheds, particularly those in the more arid sections of the country. 



The effects of changes in land use and treatment on sediment pro- 

 duction from watersheds have been determined in a few representative 

 cases by repeated reservoir-sedimentation surveys. 



In the Sangamon River watershed above Decatur, Illinois, more than 

 90 percent of the sediment is coming from sheet erosion. The rate of 

 sediment production averaged 20 percent higher during the 10 years 

 from 1936 to 1946 than during the preceding 14.2 years as a result 

 of the greater use of land for intertilled row crops from 41 to 60 percent 

 of the total watershed area. The writer and collaborators (1947) 

 have estimated that, if farmers would adopt the recommended rota- 

 tions, use contour farming, strip cropping, terracing, and other practices 

 where needed in accordance with the capabilities of the land, the rate 

 of sediment production from the area would be reduced by 62 percent 

 from its rate in 1946 without reducing the level of net farm income. 



By reforestation and gully control on 83 percent of an 890-acre area 

 of rolling and badly eroded land above the municipal reservoir at 

 Newnan, Georgia, the rate of sedimentation was reduced 62 percent 

 during the period 1937-1945, as compared with the earlier period 1925- 

 1937 (Brune, 1947). The reduction is progressively increasing as the 

 watershed-treatment work becomes more fully effective. 



On the 62-square-mile watershed of the municipal reservoir at High 

 Point, North Carolina, soil-conservation measures applied on about 35 

 percent of the total acreage caused a reduction of 24 percent of the 

 rate of sedimentation for the period 1934-1938, as compared with the 

 earlier period 1928-1934 (Brune, 1947) . Still other records have been 

 cited by the writer (1944) , and more data are now being obtained. 



From the evidence now available, it is concluded that in the prin- 

 cipal agricultural areas of the United States present rates of soil erosion 

 could be reduced 50 to 75 percent or more by proper land use and treat- 

 ment without decreasing the net agricultural income from the land 

 (Brown, 1948) . Rates of sediment production, as measured at a given 

 point in a stream system, could be reduced even more by watershed- 

 treatment measures that provide for control of channel erosion and 

 selective deposition of erosional debris at many locations within the 

 watershed. The limitation in controlling sediment production in the 

 semi-arid to arid, non-agricultural regions of the United States is more 

 economic than physical in that it depends primarily on structural con- 

 trol works, accompanied by rigid regulation of land for grazing. These 

 controls can be obtained, in general, only at public expense. The high 



