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mainstem reservoirs is now being released. This has increased the flow to the Lower Basin over the flows dut:illg the filling period (1963-80). Salini ty would have been much higher now without these additional releases. I--l- c.o -.J w During the initial filling, significant leaching of gypsum (calcium sulfate) was documented at Flaming Gorge [9J and Ruedi Reservoirs[10] and at Lake Mead,[11] but gypsum leaching at Lake Mead and Ruedi Reservoir has diminished. Final documentation of the long-term salt leaching at Flaming Gorge Reservoir is part of the ongoing reservoir studies. In addition to salt leaching, the reservoirs may play an important part in other major factors which influence salinity. There is strong evidence that Flaming Gorge Reservoir and Lake Powell have stored more salille water and routed the less saline spring runoff downstream from 1965 through 1980. These more saline waters were subject to bank storage, chemical precipitation, ion exchange, oxidation-reduction, and various biological activities. Sedimentation in reservoirs may influence both salinity and the mix of dissolved ions. Suspended sediment wllich is subject to mechanical degradation in a river environment may continue to release salts and exchange ions (sodium exchanged for calcium); however, once settled out in the reservoir, these salts and ion exchange capabilities may be isolated. Sediment stored in reservoirs may contain salts Wllich would have been released with continued mechanical breakdown in a riverine environment. Another possible loss of salinity in reservoirs is dlle to chemical precipitation. This possible loss in salinity was investigated by Reclamation for the two largest storage reservoirs in tile Basin, Lakes Powell and Mead. A thermal-hydrodynamics reservoir model, which incorporated chemical equilibria, was applied to each of the two reservoirs. The estimated potential for calcite precipitation (the salt that precipitates from solution first) was found to be 20,000 tons per year for Lake Powell and 40,000 tons per year for Lake Mead. These estimates r~present ~he upper limit of' potential precipitation, as it assumes that tllere is sufficient nuclei for the calcium carbonate crystallization and that reaction rate kinetics do not limit the precipitation. The combined maximum precipitation is les~ than 1 percent of the ~nnual salt load passing through the reservoirs and is significantly less than previous estimates which were based on inflow-outflow budgets using rather incomplete or inadequate data. 3. Irrigation and Increased Depletions Most of the irrigation projects that deplete water and increase salt pickup to the river were in place before 1965. Moreover, like the newly inundated soils in reservoirs, newly irrigated lands are subject to a leach-out period. In cases where lands with poor drainage stored salt, these areas were taken out of production. In addition, irrigation practices changed significantly during the 1960-80 period with canal and lateral lining, sprinkling systems, gated pipe, and trickle systems being introduced. These changes should result in reduced return flows and salt pickup. Projected water depletions through the 1965-80 period were largely unrealized; total depletions increased by approximately 12 percent. V-Lf