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o O':l 0) ---:l QUALITY OF WATER COLORADO RIVER BASIN PROGRESS REPORT SUMMARY This report presents the past, the present modified, and the future quality of water of the Colorado River down to Imperial Dam. The past is represented by a tabulation of the recorded or estimated historic con- dition at 17 quality of water stations for the 1941-68 period. The pres- ent modified condition includes adjustments of the historic condition based on the assumption that new developments completed during the 1941-68 period were in operation for the full period. The future quality condi- tion is an estimate of the situation after the r_~sently authorized de- velopments and some projects proposed for autholization are placed in operation. These effects are primarily related to mineral quality al- though other quality factors are discussed in the report. Studies of chemical trends indicate that under historic conditions the average concentration of dissolved solids of the Colorado River at Lees Ferry had about 0.75 ton per acre-foot, below Hoover Dam about 0.94 ton per acre-foot, and at Imperial Dam about 1.02 tons per acre-foot for the 1941-68 period. Under present modified conditions (that is assuming that the re- cently constructed projects were in operation for the entire period) the concentrations would have been about 0.84, 1.03, and 1.18 tons per acre- foot, respectively, at the three stations. It has been assumed for purposes of this study that the rate of pickup of dissolved solids from new irrigated lands would vary from zero to 2 tons per acre. It was also assumed no additional pickup of dissolved solids would occur for lands already under irrigation. Under future conditions, assuming negligible salinity control meas- ures, with all authorized projects and projects proposed for authorization in operation and with an assumed pickup of 2 tons per acre on the new irrigated lands, the concentrations are estimated to be 1.09 tons per acre-foot at Lees Ferry, 1.38 tons per acre-foot below Hoover Dam, and 1.70 tons per acre-foot at Imperial Dam. The depletions used in this report for the projects, both authorized and proposed for authorization together with present developments and other proposals, are estimated to be the ultimate depletions for the de- velopments listed. Other developments, as yet not identifiable, are expected to occur which will reduce the quantities of water shown for the various stations and cause some changes in concentrations from those indicated in this report.