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<br />Ditch lining or pipelines will reduce seepage losses from field supply <br />ditches and head ditches. It was assumed that seepage losses from the <br />remaining unl ined tailwater collection ditches would change from present <br />values in the same proportion that runoff from the fields under each <br />level of improvement changes from the present volume. This assumption <br />accounts for the lower values of salt loading from ditch seepage shown <br />for progressive levels of improvement. <br /> <br />To achieve full potential reduction, two conditions must be satisfied. <br />First, recommended management practices must be followed to a high <br />degree of precision; and, second, all of the recommended improvements in <br />the irrigation system must be installed. A discussion of these con- <br />ditions is included in the section on implementation. <br /> <br />In addition to on-farm irrigation, erosion and subsequent sediment yield <br />has been identified as a significant source of salt delivered to the <br />Colorado River. Table 20 shows the amount of sediment and salt yield to <br />the Colorado River from (1) sheet, rill, gully, and streambank (less <br />that 5 feet in height) erosion and (2) erosion from streambanks greater <br />than S feet in height. <br /> <br />The sediment and salt contributions from four contributing areas are <br />tabulated below. It should be noted that the Salt Oesert area, con- <br />taining exposures of the Mancos Shale formation, is estimated to <br />contribute nearly half of the soluble salts produced from natural <br />sources in the Grand Valley. <br /> <br />Adding the salt load of 300,000 tons picked up by irrigation return <br />flows to that from upland erosion results in 380,000 tons being added <br />annually to the Colorado River. <br /> <br />Salt load reductions attributed to improvement of privately owned range- <br />land are shown in Table 21. The volume of runoff will be reduced by 26 <br />percent and the peak discharge flow rate will be reduced by 32 percent. <br />The reduced volume of runoff will affect the salt yield from sheet, <br />rill, and gully erosion shown in Table 20. The peak flow rate has an <br />effect on the salt yield from erosion of the higher streambanks. <br /> <br />1655 <br /> <br />51 <br />