Ch. 22] CAUSES AND RATES OF SEDIMENT PRODUCTION 



381 



sites, and flood-plain lands. In some watersheds, alteration of natural 

 drainageways by deepening and straightening, by construction of dams, 

 diversions, jetties, and levees, and by regulation of stream flow has 

 further aggravated channel erosion and sedimentation. 



Several lines of evidence indicate that man-induced erosion has in- 

 creased the sediment load of streams in the humid areas of the United 

 States in the order of 25 to 100 times, and of most streams in the arid 



*-. 5,000 



£ 



— 500 — 



I 



Southern 

 Piedmont 



| Measured rate 

 = Probable rate 



Upper Mississippi 

 Valley 



Western Gulf 



Southwestern 

 United States 



Drainage Area (square miles) 

 Fig. 1. Annual sediment production rates. 



and semi-arid areas by probably at least 2 to 4 times (Brown, 1948). 

 Present rates of sediment production have been determined for sev- 

 eral hundred watersheds in the United States by sampling the sus- 

 pended load of streams and by measuring sediment accumulation in 

 reservoirs (Brown, 1945b; Brown and Thorp, 1947; Brune, 1948; Eakin, 

 1939; Faris, 1933; Fippin, 1945; Fortier and Blaney, 1928; Howard, 

 1947; Stevens, 1936). The rates have been found to vary over a wide 

 range for watersheds ranging in size from a few acres to drainage areas 

 of more than 100,000 square miles. In general, the smaller the water- 

 shed, the greater is the range, as shown in Fig. 1. This could be antici- 

 pated because smaller watersheds have greater contrasts in the factors 

 that influence sediment production, such as average slopes, drainage 



