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~ w CD CD Program Evaluation Background Colorado River salinity concentrations fluctuated widely over the period 1941 to 1993. Generally, salinity concentrations decrease in periods of high flows and increase in periods of low flows. Although high flows in the period 1983 to 1987 temporarily lowered salinity levels in the system, levels currently are rising. Figures 2, 3, and 4 show the annual flows of the Colorado River below Parker Dam, below Hoover Dam, and at Imperial Dam, respectively, and the corresponding annual salinity concentrations. Figures 5, 6, and 7 provide a historical perspective, the numeric criterion, and the projections of salinity below Parker Dam, below Hoover Dam, and at Imperial Dam, respectively, without further salinity control measures. Without the recommended controls, the salinity at all three stations is expected to increase significantly over the next 20 years. About 1.375 million tons of salt per year must be removed from the Colorado River system by the year 2015 to maintain TDS levels at the numeric criterion at all three stations. Projects that control about 262,000 tons per year have been completed. The following salinity control projects, or portions of them, are removing the approximately 262,000 tons of salt annually from the river sY8tem: Meeker Dome, Las Vegas Wash, Grand Valley, Uinta Ba8in, Big Sandy River, Lower Gunnison, and McElmo Creek Units as well as BLM well plugging. (See table 1.) Projections of future salinity levels in the Colorado River (shown in figures 5, 6, and 7) were derived from 78 sequences of historically based hydrology. Depletion projections were developed jointly by Reclamation and the Forum. Moderate variations in the salinity levels-in impoundments like Lake Powell and Lake Mead and at Imperial Dam-can be attributed to several factors, including water demands, weather, and 8alinity control measures. However, salinity levels at Hoover Dam and below are very sensitive to the following two factors: · Accumulated reservoir inflow and resulting high reservoir storage.-Whenever reservoir inflow is significantly greater than normal, dilution generally occurs within the large reservoirs of Lakes Powell and Mead. · Reservoir discharges.-Whenever riverflows are low, 8alinity concentrations are high; whenever riverflows are high, salinity concentrations are low. Very rapid changes in salinity levels can be ob8erved when these two conditions exist at the same time. For example, when: (1) previous reservoir inflows have been high for several seasons and (2) reservoir discharge has been above average, very low salinity levels can be expected, as in 1986 (less than 600 mg/L). Conversely, high 8alinity levels can be expected when reservoir inflow has been low for several seasons and the reservoir di8charge has been at a minimum. Because of the vast water storage behind Glen Canyon and Hoover Dams, Upper Basin salinity control projects implemented in any given year do not begin to reduce salinity levels at Imperial DllIl1 until many years later. This time lag is recognized when scheduling project implementation to achieve desired results. The Program The salinity control plan is designed to maintain the average salinity concentration ofthe Colorado River at or below the numeric criterion at the three stations (Hoover, Parker, and Imperial Dams) without impairing the development and use of compact.apportioned waters in the Colorado River Basin. The Basin-wide salinity control program is designed 7