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<br />PART V. HISTORICAL SALINI'IY CONDITIONS <br /> <br />r-- <br />-J <br />c.n <br />(.L <br /> <br />A. Quality of Water Records <br /> <br />Salinity in the Colorado River Basin is monitored at 20 key stations. <br />The average concentrations and loads were determined on a flow weighted basis <br />using daily data whenever possible. Salt loads and concentrations were <br />generally calculated from daily conductivity and flow records using methods <br />developed jointly between Reclamation and the USGS [8]. <br /> <br />Historical streamflow, salinity concentrations, and salt load data at 20 <br />key stations for January 1941 through December 1985 are presented in Tables 1 <br />through 20 at the end of this report. Figure V-I shows the historical <br />salinity at Imperial Dam. <br /> <br />B. Historical Salinity <br /> <br />Salinity concentrations at Imperial Dam decreased steadily from 1970-79, <br />dropped notably in 1980, increased sharply in 1981-82, and dropped again in <br />1983-84. The 1970-80 salinity concentrations show the buffering of annual <br />fluctuations in salinities due to the effect of nearly 50 million acre-feet of <br />reservoir storage. With the reservoir storage in the Colorado at near <br />capacity, discharges from Hoover Dam increased from 7.7 million acre-feet in <br />1979 to 11.1 million acre-feet in 1980 {see Figure v-2l, diluting the salinity <br />at Imperial Dam temporarily. With more normal flows in 1981 and 1982, the <br />salinity rebounded. Higher releases from Hoover and Glen Canyon Dams in 1983 <br />and 1984, combined with lower salinities in storage, caused salinity at <br />Imperial Dam to drop again. With the nearly 50 million acre-feet of high <br />quality water in storage and the relatively high runoff in the Basin, <br />salinities at Imperial Dam remained low through 1986. Under more normal <br />conditions, salinity is expected to increase quickly back to 800 rng/L or more; <br />however, as long as the runoff remains high, salinity will remain low. <br /> <br />C. Factors in Salinity Trends <br /> <br />The downward fluctuation of salinity at Imperial Dam during the 1970's is <br />within the expected range and was simulated using the Colorado River <br />Simulation System (CRSSl, a computer model of the Colorado River Basin. <br />Several factors complicate the analysis of the decline and leveling off of <br />salinity concentrations. Most of these are modeled using CRSS and include <br />variations in runoff, reservoir storage, reservoir operations, salt pickup, <br />and depletions due to development of the Basin. These, and other factors, <br />which may cause shifts in salinity, are discussed in the following sections. <br /> <br />1. Hydrologic Conditions <br /> <br />The salinity concentration in rivers generally decreases with <br />increased flow on an annual basis. Years of lower flows are characterized by <br />higher salinity concentrations than years of higher flows. Combining this <br />characteristic with the lag time in the reservoir system because of storage <br /> <br />v-I <br /> <br />- <br />