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2367 than at present because the level of the river was approximately 7 feet higher at the Reservation than at present. The flooding acted to flush accumulated salts from the flood plain soil. In addition, occa- sional high water in the 1940's and 1950's fluctuated ground-water levels, which also aided in flushing accumulated salts. For estimating purposes it was assumed that the salts in soils under natural vegeta- , tion in 1974 accumulated during the last 35 years. The vegetation in the undeveloped area south and east of Poston had an estimated consumptive use of 2 acre-feet per acre each year which had a salt content estimated at 1. 2 tons per acre-foot [4]. Because of the concentrating effect of water use, it was estimated that approximately 30 percent of the salts formed insoluble precipi- tates and the rest was susceptible to flush out when the area was initially irrigated. This produced an accumulation of 59 tons of soluble salt per acre under vegetation as of 1974. The quantity of salt per acre was adjusted for lands that were cleared prior to 1974 or that had a greater rate of evapotranspiration than 2 feet per year. The time required for soluble salt from new land to reach the river was estimated according to the following conceptual relation- ships. Soluble salt is flushed from the root zone, flows through the ground-water aquifer, and subsequently discharges into surface drains. Most of the salt leaves the root zone during the first 3 to 5 years but some takes years longer. After reaching the water table, some of the salt emerges relatively quickly but some travels a longer path deep in the ground-water aquifer. The combined effect of these mechanisms is to spread the salt discharge over a period of years. A cursory evaluation of each mechanism was made to arrive at a series of percentages which, when multiplied by the tonnage of salt in a tract of new land, would produce a salt discharge for each future year starting with the initial year of irrigation. The resulting series of percentages provided for about half of the salt to reach the drains in the first 8 years, and it was assumed the rest of the salt would reach the drains during the following 42 years in order to arrive at a round number of 50 years for the process. Estimates were then made of annual salt discharge from new lands in the recent past by applying these percentages to the tonnage 18