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<br />t.:::) <br />c.." <br />Co0 <br />""l <br /> <br />BASIC SI'UDIES <br /> <br />less water downstream, changes in its chemical composition, and a greater <br />burden of some saline constituents downstream. Diversions out of the ba- <br />sin have taken both water and dissolved salts from the river system; in <br />the headwaters relatively small amounts of salts are diverted compared to <br />amounts diverted in the lower basin. Reservoirs, by mixing the fresher <br />flood flows with the more saline low flows, have greatly reduced the <br />natural variation in river water salinity. <br /> <br />Since Lake Mead reached its highest levels in 1941, there have been <br />extended periods of higher than average salinity interspersed with periods <br />of lower salinity. This has raised questions as to whether the average <br />salinity of Colorado River water is increasing or is likely to increase. <br />Future river salinity like future streamflow can be predicted only from <br />statistical records of past salinity and from knowledge of the changes in <br />the river caused by impoundment, diversions, and use of the water. <br /> <br />The past records of water quality of the Colorado River from Lees <br />Ferry downstream have been studied to provide: (1) estimates of ionic <br />concentrations for periods of missing record; (2) weighted-average anal- <br />yses for full years for which averages had previously been computed from <br />partial flow records; (3) estimates of ionic concentrations of major con- <br />stituents not included in chemical analysis programs for some years; and <br />(4) computation of ionic loads in both tons and tons equivalent for major <br />constituents for five key points on the river for the period 1941-1964. <br />The results of this st,ud,y define the variations in salinity which have oc- <br />curred on the Colorado River both from year to year and from point to <br />point downstream, and indicate the aggregate amount of soluble mineral <br />salts gained or lost in successive reaches of the river. Thus they serve <br />as a guide to probable future concentrations under variations in both cli- <br />mate and development. <br /> <br />Variations of ionic concentrations in Colorado River water can be il- <br />lustrated by comparing annual weighted-average concentrations at different <br />locations. Selected annual weighted averages for Colorado River at Grand <br />Canyon, below Hoover Dam, and above Imperial Dam are shown in Table A. <br />Although the weighted averages in the table represent water years and <br />therefore are not strictly comparable to calendar years on a year-by-year <br />basis, the data illustrate the concentration of the various ions and the <br />pattern of change. <br /> <br />The Grand Canyon record was selected as the chief index of annual <br />weighted-average variations because continuous water quality records were <br />obtained for many more years at Grand Canyon than at any other point. The <br />period 1926-62 was one of fairly stable use of water upstream from Grand <br />Canyon; thus, this period serves as a "normal" for the river. For the. pe- <br />riod 1926-62, the annual weighted-average concentrations at Grand Canyon <br />generally have an approximate inverse relationship to the runoff. Since <br />1962 the salinity regimen of the Colorado River at Grand Canyon has been <br />changed by storage in Lake Powell. Since storage began the period of <br /> <br />28 <br /> <br />