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Program Evaluation ...... W -...l N Background Colorado River salinity concentrations fluctuated widely over the period 1941 to 1991. 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. Figure 2 shows the annual flows of the Colorado River at Imperial Dam and the corresponding annual salinity concentrations. Figure 3 provides a historical perspective, the numeric criterion, and the projections of salinity at Imperial Dam without further salinity control measures. Without the recommended controls, the salinity at Imperial Dam is expected to increase significantly over the next 20 years. About 1.5 million tons of salt per year must be removed from the Colorado River system by the year 2010 to maintain TDS levels at the numeric criterion of 879 mg/L at Imperial Dam. Projects that control about 230,000 tons per year have been completed. Therefore, an additional 1.3 million tons per year needs to be controlled. The following salinity control projects, or portions of them, are removing the approximately 230,000 tons of salt annually from the river system: Meeker Dome, Las Vegas Wash, Grand Valley, Uinta Basin, Big Sandy River, Lower Gunnison, and McElmo Creek Units and BLM well plugging. (See table 1.) Projections of future salinity levels in the Colorado River (shown in figure 3) were derived from 15 sequences of historically based hydrology. Depletion projections as of January 1990 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 salinity 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, salinity concentrations are high; whenever riverflows are high, salinity concentrations are low, Very rapid changes in salinity levels can be observed 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 Gess than 600 mg/L). Conversely, high salinity levels can be expected when reservoir inflow has been low for several seasons and the reservoir discharge 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 Dam 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 of the Colorado River at or below the numeric criteria 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 to offset salinity increases caused by man's development of the States compact-apportioned 7