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. . concentrated by the loss of fresh water, which otherwise would dilute the salt, to phreatophytes and to Burface evaporation. The salinity of the river has always increased from its head- waters to its mouth. However, the incresse has become more pronounced and has accelerated with man's development of the river system. The river supplies water for two snd one-half million irrigated acres and various other industrial and municipal needs within the Colorado River Basin. Also, enough fresh water .is exported from the basin to supply about 12 million people with full or partiai domestic supplies and to irrigate hundreds of thousands of acres of farmland, and this export adds to salinity concentrations in the Colorado River. The processes which increase salinity--salt loading and salt concentrating--are affected by development as well as by nature. The major man-made contributor of salinity is irrigsted agriculture which both loads and concentratea salts. Of the 35.3 percent of the salt load attributable to man's activities, nearly all (34.3 percent of the total salt load) is linked to irrigated agriculture. The remaining contribution from municipal and industrial sources2 is being partially brought under control by the National Pollutant Discharge Elimination System (NPDES) imposed policy of "no salt return" .whenever practicable. As further development on the Colorado River occurs it is apparent that, since all use increases salinity, control measures must apply to natural as well as man-caused sources of salinity. To 2Municipal and induatrial sources contribute 1.4 percent of total salt loading, but 0,4 percent of salt load is exported in the water used for out-of-basin diversions. Source: EPA, The Mineral QualIty Problem, p. 15. 33 ! 1438 'j .! I I i " I j ;,