376 golze. PROBLEMS OF IRRIGATION CANALS [Ch. 21 



amount of the material that is carried now by water passed through the 

 desilting works. 



The construction of Imperial Dam and the desilting works cost a 

 total of about $10,000,000, of which $4,526,000 was charged to the de- 

 silting works alone. Amortizing this amount in 40 years, without in- 

 terest, develops an annual cost of $113,150. The annual operation and 

 maintenance costs of the desilting works, in the four years 1945 

 through 1948, averaged $69,000. Assuming this figure as an average, 

 the total annual cost of the Imperial Dam desilting works would be 

 less than $200,000, which represents a saving of at least $500,000 per 

 year resulting from this construction. 



RESEARCH NEEDED 



Sedimentation in canals is an unwelcome expense on the water users 

 served by irrigation projects of the West. To reduce this burden as 

 much as possible, further research is needed into both the sources and 

 causes of silt and the methods of removal where elimination at the 

 source is not practical. These are fields for both the geologist and the 

 several branches of engineering. 



More information is needed about the relation between the condition 

 of the watershed and the amount of silt carried by streams. Al- 

 though the desert areas of the Southwest are often considered the 

 major sources of silt, more attention must be given to the burned and 

 overgrazed areas in all parts of western United States. The U. S. 

 Forest Service has done considerable work in determining the effect 

 of forest fires and overgrazing on soil erosion and stream siltation. 

 The correlation of those data with the percentage of silt in irrigation 

 water on its arrival at the project is still to be developed. If the direct 

 relationship between denuding of forested areas and the cost of silt 

 removal from irrigation canals can be shown, the farmers on irrigation 

 projects will be more active supporters of forest-conservation programs. 

 It should be possible to develop by research, for example, the actual 

 cost of a forest fire or overgrazing to irrigation systems dependent on 

 the watershed affected, and to show the actual increase in cost of 

 removal of silt arising from such causes. Information of this kind 

 would also support a program for reforestation of burned-over areas 

 and should contribute materially to securing congressional and state 

 appropriations for that purpose. 



The design of desilting basins should be improved to provide for re- 

 moval at a minimum of cost of silt suspended in irrigation water. 

 The trapping of silt in gravity basins is essentially a process of shift- 



