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I. INTRODUCTION N en . H;>.'. <:.D There appear to Ibe many impacts resulting from changes in salinit~ of the waters of the Colorado River. By far theimost significant impact is the economic 'effedt of using water with a high concentration of'dissolved solids. It is imperative that evaluation of measures to correct this problem is well-founded and based on sound concepts and information. To achieve this end. the usepf Erconomic tools is increasing to more effectively match the problem to policy-decisiorl' criteria. Economically. the problem is that tpe well-being of some users of the river conflicts with the well-being of other users. The following a6alysis uses both primary and secondary data (0 develop improved estimates of the economi~ impacts. Resulting estimates are intended to b,e used as benefit values as one measure in evaluation of salinity control proposals and'asa quantification of the salinity-related externalities associated with proposed water resource development projects. II. THE PROBLEM SETTING The waters of the Colorado River are progressively increasing in salinity. The principal dissolved constit~ents in the river waters are the cations: calcium) magnesium. and sodium; and the anions: sulfate. chloride. and bicarbonate. These. and small amounts of other dissolved constituents, are commonly referred to as salinity. Increasi6g concentrations of dissolved mineral salts threate.n to become a major economic problem for users of Colorado River water. At the headwaters. the average salinity in the Colorado River i$ less than 50 mg/L (milligrams per liter) and!progressively increases downstream until. at Imperial Dam. the present annually weighted condition (1976) is 823 mg/L. Proj:ections of the future salinity suggest values of about 1200 mg/L at Imperial Dam by the year 2000. Should such salinity increases be realized. substantial economic impacts would tie incurred by both agricultural and municipal u$ers in the Lower Basin. Salinity increases result from ,two processes: salt loading and salt concentrating. Salt loading increases the amount of salt for a given amount of water. and salt concentrating decreases the amount of water for a given amount of salt. Salt loading is the addition to the river system of mineral salts from natural and manmade sources. Salt concentrating is the rise in salinity through: streamflow depletions which concentrate the salt burden in the river system into a lesser volume of water. The amount of salt in water directly influences the utility of the water. and increased salt. concentrations typically have a detrimental' impact on water users. Such detriments include decreased productivity and/or increased production costs for both agricultural and industrial water users. In household uses. the detriments include reduced life of water-related equipment. lowered palatability of drinking water. and at higher concentrations of salinity, adverse health effects. Salinity of waters in the Colorado River Basin can be expected to continue to increase as basin development goes forth in the absence of any action to control salinity. The seven Colorado River Basin States have joined with the Environmental Protection Agency in the development of salinity standards for the river. They have been instrumental in securing passage of the Colorado River Basin Salinity Control Act (Public Law 93-320), which authorized four salinity control units identified by the. Colorado River Water Quality Improvement Program. Additional investigations are proceeding on 13 other units by the Water and Power Resources Service. Expected growth of municipal, domestic. industrial. irrigation. and recreational uses of water will add mineral constituents and, deplete the flow of the Colorado River. All phases of the basin economy are expected to expand in the future. requiring increased water use. As the entire basin develops. the salinity problem will be further aggravated. At present, salinity adversely affects the water supply of a population exceeding 10 million people and over 405 000 hectares (1 million acres) of irrigated land in the Lower Colorado River Basin. Salinity also affects water users in Mexico. As salinity levels rise. the net economic return to agriculture decreases. In addition, municipal and industrial water users experience i';. .