368 golzb. PROBLEMS OF IRRIGATION CANALS [Ch. 21 



sometimes, even plane-table surveys of the chief areas of erosion 

 which have been isolated by the aerial surveys. 



When surveys have isolated the sources of silt, it becomes the per- 

 sonal problem of the geologist and the engineer. A determination 

 must be made of the class of material being eroded (rock, soil, sand, 

 etc.), cause of erosion (wind, rain, excessive irrigation, etc.), the 

 geologic history, and probable geologic future. Accompanying the re- 

 port of the geologist there should be an engineering report citing pos- 

 sible plans for controlling or eliminating erosion by dam construction 

 (major or check dams), channel controls, or other means. The engi- 

 neer's report must then estimate the cost so that it can be balanced 

 against downstream benefits to determine whether the work is economi- 

 cally justified. 



It may be well to point out here that watershed surveys and cost- 

 benefit reports are seldom undertaken to improve sedimentation of 

 canals. Usually such surveys are undertaken to determine flood- 

 control benefits. With the increasing cost of silt removal from canals 

 more attention should be given to the condition of watersheds than 

 heretofore. 



Ownership of the watershed is a major factor in any solution. In 

 the western states a watershed that contributes to the silt of the irri- 

 gation systems is most likely to be in federal ownership. However, 

 this ownership may very properly be divided among several bureaus 

 of the Department of Agriculture, the Department of the Interior, 

 and war and defense agencies. In addition, there is a possibility that 

 large private ranches may be interspersed among the federal tracts. 

 If erosion is occurring on these lands to a serious extent, a cooperative 

 effort among all owners, public and private, is essential to overcome 

 the problem. 



The development of a cooperative agreement and the supervision 

 of its operation should be the responsibility of the principal land- 

 owner involved or of the government agency concerned with that type 

 of activity. Adequate funds are important, as lack of funds is the 

 primary reason why so little work has been done in this field in the 

 past. Congress regularly appropriates funds for soil-erosion control 

 on both private and public lands, and the wise application of these 

 funds to treatment of watersheds contributing silt inflows into irriga- 

 tion systems can be a material factor in reducing or eliminating such 

 silt. Good geologic and engineering reports can do much to insure 

 the availability of necessary funds. 



In the same way, major contributions of silt from lands adjoining 

 irrigation systems should be isolated and examined. Aerial photogra- 



