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~ o ~ O~ Implications for Salinity Control Congress authorized the U.S. Department of Interior (Bureau of Reclamation) to proceed with construction of four salinity control projects under Title II of the "Colorado River Basin Salinity Control Act" (PL 93-320) of 1974. These 'included: the Grand Valley and Paradox Valley units in Colo- rado, the Las Vegas Wash unit in Nevada and the Crystal Gey- ser unit in Utah [3]. The Crystal Geyser unit has since been dropped from further consideration, but implementation of the others is proceeding on schedule. Collectively, these salinity control projects will decrease TDS concentrations at Imperial Dam by 65 mg/l [4]. Numerous other projects are in the planning or feasibility stage, and, if all are imple- mented, these would reduce TDS by 130 mg/l at Imperial Dam [4] . , Control of point sources is obviously an effective method of reducing TDS concentrations, and this approach app"ars to be warranted in view of predictions that TDS could increase to'1150 mg/l by year 2000 [4]. These predic- tions, however, are based on models that assume TDS concen- trations are inversely related to flows. This was the case during pre-impoundment periods, and the assumption is per- haps still valid for extreme variations in flow. However, flow vs. TDS relationships have been highly modified by the large impoundments. The concentrations of principal ions in the Colorado River are now altered significantly by mineral dissolution and precipitation and evaporation processes occurring in Lake Powell and Lake Mead. Rates of calcite precipitation are closely linked to rates of phytoplankton productivity. Inhibition of calcite precipitation occurs to varying degrees in both impoundments. Rates of gypsum and halite dissolution have been decreasing in Lake Mead since early impoundment and will probably decline even more in the future. A similar trend has been predicted to occur in Lake Powell [12]. Although net water losses in Lake Mead were higher, apparently due to increased bank storage during the 1970-79 period, evaporation rates appear to have been reduc- ed considerably by cold-.wa ter discharges from Glen Canyon Dam [19,20]. It is estimated that this has reduced average l'DS concentrations by' 9 mg/l [20]. The impoundments have thus caused numerous changes in ion and TDS concentrations that cannot be modeled by simple flow vs. TDS relationships. ,Rates of calcite precipitation, mineral dissolution, and evaporation must be incorporated into the models, if they are expected to have any predictive value. Moreover, the whole concept of controlling TDS point sources seems illogical in view of the natural ion composi- tion of the river and the effect that these have on various beneficial uses. Sulfate comprises nearly one-half the TDS 14