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22 Chapter 5-HistoricaJ Salinity Conditions 3700 ~ ~ 3700 3600 3600 .- .- o.l o.l :l 3500 :l 3500 1985 1988 .... .... -- -- N ; 3400 ~ 3400 - t-- o.l <<00 I> Q) 3300 .! 3300 - CO ra1 ra1 3200 3200 1985 1988 3100 3100 200 300 400 500 600 700 800 200 300 400 500 600 700 800 Salinity (me/L) Salinity (mg/L) Figure 4.- Salinity at Hite (left) and Wahweap (right) stations in lake Powell. In addition to salt leaching, the reservoirs may play an important part in other major factors that influence salinity. Strong evidence exists that Flaming Gorge Reservoir and Lake Powell have stored more saline 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 oi dissolved ions. Sediment which is subject to mechanical degradation in a river environment may continue to reiease saits 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 that would have been released with continued mechanical breakdown in a riverine environment. Another possible loss of salinity in reservoirs is due to chemical precipitation. This possible loss in salinity was investigated by Reclamation for the two largest storage reservoirs in the 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 represent the upper limit of potential precipitation, as it assumes that there are sufficient nuclei for the calcium carbonate crystallization and that reaction rate kinetics do not limit the precipitation. The combined maximum precipitation is less than 1 percent of the annual 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. 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 the introduction of canal and lateral lining, sprinkling systems, gated pipe, and trickle