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PART V HISTORICAL AND PRESENT SALINITY CONDITIONS e salinity concentrations. Most of these are modeled using CRSS and in- clude variations in runoff, reservoir storage, reservoir operations, salt pickup, and depletions due to development of the basin. These and other factors, which may cause shifts in salinity, are discussed in the following sections. 1. Hydrologic Conditions The salinity concentration in rivers generally decreases with increased flow on an annual basis. Years of lower flows are character- ized by higher TDS concentrations than years of higher flows. Combining this characteristic with the lag time in the reservoir system because of storage suggests the decline of TDS concentrations may have been in part caused by the transition from a relatively drier period (1955-65) with an annual virgin (undepleted) flow of 13.23 million acre-feet at Lees Ferry, to a relatively wetter period 0965-75) with an annual virgin flow of 14.76 million acre-feet. This is an increase in the flow of approximately 10 percent and may be responsible for a signiflcant por- tion of the 9 percent decrease in TDS concentration observed at Imperial Dam. 2. Reservoir Effects e One of the most significant changes which has occurred to the salinity of the Colorado River is due to the regulation of the natural flow of the river basin. Due to the effects of dilution, the natural, annual variation of the river flow caused salinity to vary inversely to flow. Low flow periods have much higher salinities than high flow periods. This seasonal variation in both flow and salinity has been greatly reduced by the regulation of the basin. The period of 1963-80 represents the most significant period of reservoir storage in the history of water development on the Colorado River. Storage in Flaming Gorge Reservoir, Lake Powell, and Lake Mead increased from less than 20 million acre-feet in 1963 to over 50 million acre-feet by 1980. The spill of Glen Canyon Dam in 1980 ended the ini- tial filling of the major reservoirs on the Colorado River. During the initial filling, significant leaching of gypsum (calcium sulfate) was documented at Flaming Gorge[6] and Ruedi Reser- voirs[7] and at Lake Mead,[8] 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. e In addition to salt leaching, the reservoirs may play an im- portant part in other major factors which influence TDS. There is strong evidence that Flaming Gorge Reservoir and Lake Powell have stored higher TDS water and routed the lower TDS spring runoff downstream from 1965 through 1980. These higher TDS waters were subject to bank storage, chemical precipitation, ion exchange, oxidation-reduction, and various biological activities. 25