Ch. 21] CONTROL OF SILTING 369 



phy is not ordinarily required, as most lands are under cultivation 

 and sources of silt are found in wasteways, drain ditches, or natural 

 creeks that are carrying waste or storm water from land being eroded. 



Natural stream channels, if overloaded by irrigation waste water or 

 runoff from storms, may become major sources of silt through erosion 

 of the channel cross section. In the western United States, dry land 

 subject only to natural precipitation may change its physical charac- 

 teristics under irrigation and the rise of the water table. Geological 

 reports are invaluable in appraising the probable erosion action of 

 land proposed for irrigation or being irrigated for the first time. 

 Eroding channels, which may have a major effect on increasing sedi- 

 mentation of irrigation canals lying below, are often a difficult prob- 

 lem to overcome, not because of lack of information but because of 

 lack of responsibility and funds. Elimination of erosion of these 

 waterways is not ordinarily the responsibility of the organization that 

 operates the irrigation system. Community action or a soil-conserva- 

 tion district may be required to begin corrective measures. Again 

 good reports of ground conditions are essential. Elimination of the 

 silt contributed from these sources then becomes largely a matter of 

 having adequate funds to control or eliminate the erosion, and this 

 may be accomplished by redesign of the waterway, by placing of 

 lining or riprap, or by substituting an alternate waterway not subject 

 to excessive erosion. 



Bank sloughing is a condition that contributes silt within the canal. 

 This condition is likely to occur in any area where the velocity of the 

 water exceeds the original design limit so that the water contains 

 sufficient power to cut into the material forming the banks of the 

 canals. Such action will also occur where the bank material is less 

 resistant than that used in the theoretical design. As the water 

 cuts into the canal bank, portions of it above the water line are left 

 suspended, and they fall into the canal when the weight of the sus- 

 pended portion exceeds the vertical shearing strength of the bank 

 material. The portions that fall into the canal either dissolve and 

 become suspended material (silt) floating downstream or remain ap- 

 proximately at the place where they fell, as chunks of eroded ma- 

 terial, impeding the free flow of water. Lining of canals to above 

 the high-water line is the remedy for a condition of this kind. Lining 

 to prevent seepage may be accomplished with inexpensive materials 

 such as silt itself, whereas lining to prevent erosion must be of a more 

 substantial material such as asphalt or concrete. The latter class of 

 materials, which are not subject to erosion, are also relatively water- 

 tight. Hence lining with materials of this class performs two functions 



