Ch. 21] CONTROL OF SILTING 367 



Some idea of the effect of silt seals in irrigation canals can be 

 gained from the experience of the Yuma project in Arizona on the 

 lower reaches of the Colorado. Records of the project show that 

 for the 10-year period 1927 to 1937 an average of 13.5 percent of 

 all water delivered to the Valley Division was recovered in the drains 

 and pumped to Mexico at the International Boundary. Beginning in 

 1938, there was a marked increase, to 18.9 percent, with a continued 

 increase to 24.5 percent in 1947. The most likely cause of the in- 

 crease is seepage from unlined canals, arising from erosion of the old 

 silt seals by the change in silt content of the water supplied from the 

 Colorado after the closing of the gates at Hoover Dam in 1935. The 

 silt content changed from finely divided soil particles to coarse granu- 

 lar material, picked up from the river bed below Boulder Canyon. 



That silt is a fertilizer is generally recognized. The periodic over- 

 flowing of the great rivers of the world has long been a source of 

 nutriment to the flooded lands — whether they were in the valley of the 

 Nile, Mississippi, or Missouri. Only on silt-free rivers like the Co- 

 lumbia is the fertilizing feature not evident. Facts and figures are 

 not readily available, but it is known that removing silt from irriga- 

 tion water formerly having a high silt content results in forcing the 

 affected farmers to increase their use of commercial fertilizers to main- 

 tain crop yields. 



CONTROL OF SILTING 



With this understanding of the effect of silt in irrigation canals, 

 the problems to be considered are restated: (1) How can the inflow 

 of silt into canals be reduced or eliminated, and (2) how can the cost 

 of removal of silt be reduced? 



The reduction or elimination of the inflow of silt into the canal 

 must go back to the sources of silt, which are the watershed, canal- 

 side lands, including tributary waterways and sloughing of the canal 

 banks. 



The steps that could be taken to overcome the first problem are 

 reviewed in the same order. To reduce the inflow of silt or to elim- 

 inate it from the watershed involves a number of actions. 



As a first step, the sources from which the silt is originating must 

 be determined. This requires a survey of the watershed which, for 

 some irrigation areas, may involve large areas of semi-arid desert, 

 mountainous, and completely arid desert country. Aerial reconnais- 

 sance maps are the best source of information, followed by close- 

 order aerial photography of the principal areas contributing silt, and, 



