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!inity in the Colorado River Basin is the res!r two basic processes: 1) salt loading, increasing the total salt burden carried by the riveri 2} salt concen- trating, concentrating the salt burden in a lesser volume of water due to a reduced water supply. Both processes may result from both natural and man-made conditions. Salt loading occurs naturally from surface and ground water runoffs as well as from mineral sl?rings. 'Man-made salt loading results from irrigation return now and industrial or municipal discharges. Salt-concentration is a- natural result of stream or reservoir evaporation and consumption by ~hreatophytes and other riparian vegetation. Of the man-made increases in salinity, irrigation is the major source. It contributes to both salt loading and salt concentration. Accor~ing to the BLM in its Final Environmental Impact Statement on the Colorado River Quality Im- provement Program agriculture accounts for 46% of the total basin salt load. Pg. 1-20, Figure I-S. Diversions from the Upper Colorado River Basin, at or near the headwaters where the river's water has a very low salt content, into surrounding basins account for part of the remainder of man-made concentrations. along with in~ustrial and municipal consumptive in-basin use. '. The EPA has estimated In its 1971 report "The Mineral Quality Problem in the Colorado River Basin" that almost two-thirds of the a.verage annual salt load BlJd one-half the concentration at Hoover Dam for the Ite'riod 1942-61 was caused by natural sources. Almost the same results were shown in the report ,for the 1963-66 period. The report prepared by EPA on the mineral quality problem in the Colorado River Basin, provided the basis for development of salinity standards. This report included three alternatives for salinity control in the River: 1) augmentation of basin water supply; 2) reduction of salt loads; 3) limitation of further depletion of basin water supply. The report concluded that a basin.','lide salt load reduction program appeared to be .the (most) feasible of the three salinity controls. In essence, this alternative included impoundment and evaporation of saline water from (:Ioint sources, diversion or runoff and streams around areas of high salt pic;":'up, improvement of irrigation and drainage ~ractices, improvement of irrigation conveyance facilities, desalinization of saline discharges from natural and man-made sources and desalinization of water supplies at points of use with appropriate disposal of waste brine. The report estimated that if fully implemented, the program would reduce average salinity concentrations at Hoover Dam by about 150 mg/l in 1980 and 275 mg/l in 2010. -8-