398 brown. EFFECTS OF SOIL CONSERVATION [Ch. 22 



stream-discharge velocities; hence much of the sediment load may- 

 pass through and out of the basin. 



Sufficient capacity must be maintained in domestic and industrial 

 water-supply reservoirs to assure continuity of supply during periods 

 of prolonged drought and to meet normal increases in water demand 

 over a reasonable number of years. If such reservoirs are built with 

 capacities far in excess of reasonable requirements, the additional 

 cost represents in effect prepaid insurance against loss by silting. If 

 the capacity is just about equal to the reasonable requirements, any 

 reduction by silting represents a direct damage which involves both 

 an accumulating loss of service value and a future replacement cost 

 (Brown et al, 1947). 



In recreation reservoirs, sediment creates conditions unfavorable to 

 fish life; accumulates on sandy beaches, thereby making them less 

 desirable for swimming; impedes boating; causes swamping in the 

 upper end of the lake and on the shores, thereby decreasing esthetic 

 values and often property values, and in some sections of the country 

 increasing health hazards from malaria. 



Navigation reservoirs are damaged both by shoaling of ship chan- 

 nels, which necessitates frequent costly dredging operations, and by 

 loss of storage capacity required for regulating low-water flows. 



Various methods have been used to reduce the rate of reservoir 

 sedimentation (Brown, 1944) . The most lasting solution of the prob- 

 lem is reduction of the rate of sediment production from the con- 

 tributing drainage area, provided that the reservoir is an efficient sedi- 

 ment trap and that sedimentation has not already progressed to the 

 point where the reservoir would be useless before effective measures 

 could be carried out on the watershed (Brown, 1946; Marshall and 

 Brown, 1939). The sources of sediment must always be determined 

 before an effective watershed-treatment program can be planned. 



POTENTIALITIES FOR EROSION AND SEDIMENT CONTROL 



Effective methods for reducing current rates of erosion have been 

 developed and demonstrated (Bennett, 1939). Musgrave (1947) has 

 shown that the principal causal factors of sheet erosion are: (1) rain- 

 fall, (2) slope, (3) soil, and (4) vegetal cover. By correlating a large 

 mass of experimental data, he has developed a first approximation of 

 the relative effects of the principal causal factors. For rainfall, the 

 data indicate that 



E <X Pgo 1 ' 75 



