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<br />T <br /> <br />C,"> . <br />cYj <br /> <br />~ <br />c' The impact analysis is not complete; however, the preli"ninary data indicate <br />::) withdrawal or di versions coul d be reduced about 2 mi 11 ion acre-feet or coul d <br />be utilized to provide a more adequate water supply to lands which now have <br />a deficit. It appears that improved irrigation systems and good management <br />practices could reduce salt loading to the Colorado River by about one <br />million tons. <br /> <br />The erosion and sedimentation study carried out in cooperation with the <br />Western U.S. Water Plan has given emphasis to critical erosion and sediment <br />producing lands in the West. A major consideration is the location and <br />extent of salts in the soils subject to erosion. A sediment yield map and <br />a saline soil map of the eleven Western States are being developed. Study <br />analysis involves correlatil;] high sediment producing areas with saline <br />soil areas. An analysis of sediment load, salt load and water yield at <br />selected stream gage locations will be included. Some of this analysis has <br />been 'made in the Colorado River Basin as a Dart of preparing this report. <br /> <br />Additional data were developed to complement the short term 7-year period <br />of record shown in Table 1 on salt loading in the Grand Valley. Information <br />on the Colorado River indicates that 80 to 90 percent of variance in salt <br />loading of the Colorado Main Stem is due to variance in natural streamflow <br />or runoff from the basin. It appears that the trend of decreased salt load- <br />ing in the 30-year period. 1941-70, on the Colorado is primarily resulting <br />from correlary decrease in water yield in a declining portion of the hydro- <br />logic cycle. However, other causal factors such as improved irrigation <br />practices cannot be discounted. <br /> <br />USGS water~uality data for the 1941-70 period for three gaging stations <br />~Were usecrfor this analysis: the Colorado River at Cisco, Utah; Colorado <br />River at Cameo, Colorado; and the Gunnison River near Grand Junction, <br />Colorado. Figure 5 displays the 3~year moving average streamflow which <br />represents the inflow to the Grand Valley area (Colorado River at Cameo <br />plus Gunnison Rive.~) and the outflow (Colorado River at Cisco). Figure 6 <br />displays the s~gnificant variance in salt loading with streamflow. A high <br />correlation, r = 0.78 to 0.88, exists which indicates that salt loading <br />can be predicted with a high degree of confidence by the amount of runoff <br />from the watershed. During the 1941-70 period, the variance for the 3-year <br />moving average in Colorado River streamflow at Cisco has been over 3.5 <br />million acre-feet and 3~year moving averages in correlary salt loading have <br />been 1.9 million tons. <br /> <br />Withdrawals and consumptive use on the 720,000 acres of irrigated land in the <br />Colorado River Main Stem have been relatively constnnt from year to year be- <br />cause of an abundant water supply in relation to irrigation requirements. It <br />is not expected that much of the annual variation in salt lCo1;r.~ would re- <br />sult from irrigation. <br /> <br />Figures 7, 8 and 9 display the streamflow and salt load pick up in the Grand <br />Valley area for the 1941-70 period. A high correlation in salt pick up to <br />streamflow exists. Further analysis of the relationship of salt loading to <br />erosion and sedimentation is warranted. <br /> <br />- 23 - <br />