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645,000 people. At the present rate of population increase, M&I sources will be increasing salinity by approximately 133,000 tons per year by the year 2010. However, most municipal wastes are relatively low in salt concentration in comparison with natural, industrial, and agricultural sources. Complete elimination of such waste discharges would be expensive when compared to other salinity control methods. Consumptive Use of Water Increases Salinity Addition of salts to the river system is not the only cause of increased salinity in the Colorado River Basin. The consumption (depletion) of water reduces the dilution of saline inflows to the river system, increasing the concentration of salinity. Water use is evaluated as part of the Bureau of Reclamation's responsibilities in managing the river system. The Colorado River Basin, Consumptive Uses and Losses Report[4] summarizes water use in the Basin. Table 2 summarizes the Colorado River Basin uses for 1981-85, including tributaries to the Colorado River in the Lower Basin. Work is underway to update these estimates. Table 2.-Water use in the Colorado River Basin (1981-85) Total Upper Basin use Lower Basin use (1,000 af/yr)' (1 ,000 af/yr) 812 1,255 2,312 5,101 203 841 0 30 669 4,063 3,996 11 ,290 Type of use Reservoir evaporation and channel losses Agriculture (within basin) Municipal and industrial Fish, wildlife, and recreational Transbasin exports 1 Acre-feet per year. With the exception of the Central Arizona Project (CAP), the Lower Basin has already developed most of its water supply. CAP is the last major development to deplete water from the Lower Colorado River (approximately 1.5 million acre-feet per year). Agriculture use is the single largest cause of depletions of the Colorado River. Exports, reservoir evaporation, and M&I uses also account for lesser but significant depletions. Most of the exports from the Upper Basin are made at higher elevations where the salinity concentrations are very low. This loss of high quality water results in the remaining flows downstream becoming more concentrated. Water exported from the Upper Basin during the period 1941-72 averaged about 360,000 acre-feet per year. Completion of such large projects as the Colorado-Big Thompson, Duchesne Tunnel, Roberts Tunnel, and more recent projects increased exports to about 727,000 acre-feet per year for 1976-80, with a peak in 1978 of 852,000 acre-feet. ~ 11