388 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



class is rarely an important source of the total sediment production 

 from any sizable watershed. 



The preceding classification makes no distinction between geolog- 

 ically normal and accelerated erosion. Sheet erosion and gullying 

 have always occurred on sparsely vegetated land in semi-arid regions. 

 The rate has varied almost directly with the density of vegetal cover, 

 and to a more limited extent with the type of cover (U. S. Forest 

 Service, 1936). When the density and type of cover change, whether 

 with varying precipitation or because of overgrazing and burning of 

 the cover, the rate of sheet erosion will naturally change (U. S. Forest 

 Service and Soil Conservation Service, 1940). The concept of "ac- 

 celerated" erosion, however, implies a change due to man-induced de- 

 pletion of the cover and other uses of the land. In forested or densely 

 vegetated areas, geologically normal erosion of the land slopes occurred 

 primarily by soil creep and subsequent removal of the creep material 

 by stream-bank erosion (Lowdermilk, 1934). 



Sheet, gully, and stream-channel erosion are by far the most im- 

 portant sources of the sediment load of most streams. Sheet erosion 

 generally produces a major part of the sediment load of streams over 

 broad areas that are predominantly agricultural and have more than 

 20 inches of precipitation. Sediment that causes particular problems 

 such as channel aggradation may come, however, primarily from gully- 

 ing or other sources. In most forest and range country, and in areas 

 having less than 20 inches of precipitation, gullying and stream-chan- 

 nel erosion generally furnish the greater part of the total stream load. 

 In small watersheds any of the other sources may be the most im- 

 portant. Specific data on sources of sediment in certain watersheds 

 have been previously published by the writer (Brown, 1944). 



THE SOIL-EROSION PROBLEM 



According to the U. S. Census of 1945, continental United States 

 contains approximately 1,905 million acres of land. Of this total, 148 

 million acres, or nearly 8 percent, has been classified as mountains, 

 mesas, and badlands. 



Of the remaining 1,757 million acres, which is used or is potentially 

 useful for cropping, grazing, or timber production, about 282 million 

 acres, or nearly 15 percent of the total, has been severely eroded since 

 the white man first occupied the land. On the average, more than 75 

 percent of the original topsoil has been removed by surface runoff and 

 wind action. Much of the land is severely gullied. Approximately 

 100,000,000 acres of this class of land, or about 5 percent of the total 



