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PROBLEMS AND NEEDS Colorado River Water Quality The Lower Gunnison Basin Unit area contributes about 364,000 tons of salt annually to the Colorado River, which results in an increase of about 37 mg/L at Imperial Dam. As a result of this and other salt sources, salinity concentrations in the lower Colorado River have been increasing over the long term. The Colorado River at its headwaters in the mountains of north- central Colorado has a salinity (dissolved mineral concentration) of only about 50 mg/L. The salinity progressively increases downstream as a result of water diversions and salt contributions from a variety of sources. In 1979, salinity concentrations averaged about 810 mg/L at Imperial Dam, the last major diversion point on the Colorado River in the United States. The high salt load of 10 million tons annually in the Lower Colorado River Basin adversely affects more than 17 million people and about 1 million acres of irrigated farmland in the United States. Municipal and industrial water users and irrigators experience estimated direct and indirect annual economic losses of $472,000 (January 1981 prices) for each increase of 1 mg/L in salinity at Imperial Dam. The salinity also adversely affects water users in Mexico. The losses associated with municipal and industrial use occur pri- marily from increased water treatment costs, accelerated pipe corrosion and appliance wear, increased soap and detergent needs, and decreased drinking water palatability. The Public Health Service recommends that drinking water contain no more than 500 mg/L of total dissolved solids (TDS). For irrigators, the high concentrations cause decreased yields, altered crop patterns, increased leaching and drainage require- ments, and increased management costs. Agricultural losses begin when salinity levels reach 700-850 mg/L, depending upon soil conditions and the type of crop grown. Salt loading occurs as a result of weathering and the dissolution of soluble salts in soil and substrata. In addition, salt concentration occurs due to consumptive use, such as agricultural or industrial appli- cations of water. Generally, the application of irrigation water results in increased salt concentrations because of both salt loading and con- sumptive use. The historical salt load fluctuates annually with the overall basin water supply. Without control measures and as the Upper Basin States con- tinue to develop their compact-apportioned waters, the salinity at Imperial Dam is expected to increase. As the following graph indicates, since 1949 the general trend of salinity at Imperial Dam has been upward although since 1970 the' salinity has decreased, possibly as a result of the con- struction of Colorado River Storage Project reservoirs such as Glen Canyon. This downtrend is believed to be a temporary phenomenon, and the projected r, n ., ~ c.' S 10 '~. \ ].. ( ....