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2359 The amounts tabulated for years 1968 and 1969 were estimated using data from less frequent measurements than are made under the ISSP program, and have a lower confidence level. However, they are judged to be adequate for this preliminary analysis. Figure 3 shows the quantities of diversion water and return flow for each year of record, their salinity in milligrams per liter, their salt content in tons, and the net salt discharge in tons. An attempt was made to evaluate the reasons for the variation in net salt discharge as shown on the preceding page. It was found that the years with the three relative highs were years of relatively low per acre water use, apparently involving interruptions of irriga- tion operations. In 1976 a tropical storm caused unusually heavy late summer rain, which apparently caused more of a reduction in diver- sions than drainage. Higher than average rainfall also occurred in 1970 and 1972 and the timing of rainfall may have interfered with irrigation deliveries, but this correlation is not strong. The varia- tions are discussed further in a following section of this chapter. The Boyle Engineering Corporation was retained by the Bureau of Reclamation in 1975 to gather data on irrigation efficiency and agricultural drainage, and to make a preliminary evaluation of whether water system improvements would reduce salt discharge to the river. The results of the study were presented in a report entitled "Salinity Control and Irrigation System Analysis, Colorado River Indian Reservation," August 1976. [2] Using primarily data from 1974 operations, Boyle Engineering concluded that the Reservation received 779,036 tons of salt and discharged 647,608 tons. The difference of 131,428 tons was retained on the Reservation. The various components of the water and salt budgets are on Table 1, which shows that a substantial amount of water and salt enters the Reservation in addition to Colorado River diversions. The Boyle estimate can be reconciled with the Bureau of Reclamation estimate for 1974 if certain unmeasured inflows are added to the latter. The Bureau of Reclamation ISSP data include only diversions at Headgate Rock Dam and discharges in the three drains 12