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W "l 00 W Below Hoover Dam Below Parker Dam Imperial Dam 723 mg/l 747 mg/l 879 mg/l Each of the Basin states adopted the standards for salinity as presented in the 1975 Forum report. The state-adopted water quality standards were subsequently approved by EPA. In response to Section 303(c) of the Clean Water Act, the Forum in 1978, 1981, 1984, and 1987 reviewed the standards. After each review, the Forum determined that the 1975 criteria did not need to be r$vised. The Forum also reviewed and modifi$d the plan of implementation in 1978, 1981, 1984, and again in 1987 to reflect changes in salt removal requirements to maintain the criteria. In 1990, the Forum, in response to Section 303(c), again reviewed the numeric criteria and determined that they did not need to be revised. The plan of implementation was also r$viewed and modified to reflect changes that have occurred since 1987. The plan is described in Chapters IV and V. Temporary Increases The plan of implementation asset forth in this Review is designed to remove enough salt from the river system to maintain salinity levels at or below the 1972 levels as far as it may be determined that development and/or man's activities have impacted the salinity levels. The program is not, however, intended to counteract the salinity fluctuations that are a result of the highly variable annual flows (natural variations in the hydrologic cycle). The plan of implementation for this review is based on the assumption of a long term mean water supply of 15'maf/yr, as were the 1975 Report and all subsequent Reviews. It shoUld be recognized that the river system is sub,1.;ct to highly variable annU3~ flow. The frequency, duration, and availability of carryover storage greatly affect the salinity of the lower main stem and, therefore, it is probable that salinity levels will exceed the numeric criteria in some years and be well below the criteria in others. Given the above assumptions, the average salinity will be maintained at all times at or below 1972 levels. Periodic increases above the criteria as a result of reservoir conditions or periods of below long-time average annual river flow also will be in conformance with the regulation. Wi th satisfactory reservoir conditiuils and when river flows return to the long-time average annual flow or above, concentrations are expected tr be at or below the criteria level. As shown in Figure 2, Chapter 2, the flow-weighted aver",:,;. annual salini ty concentrations can fluctuate greatly. Recent analyses have ,shown that the impact of natural variations in the 19