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o o W CJl Temporary Increases . The federal regulations provide for temporary increases above the 1972 levels if sufficient control measures are included in the plan of implementation, Should additional water development projects take place beyond those anticipated to occur before control measures are brought on line, temporary increases above the numeric criteria could result, However, these increases will be deemed to conform with the standards if appropriate salinity control measures are included in the plan, The standards require that a plan be developed which will maintain the, flow-weighted average annual salinity at or below the 1972 levels while the Basin states continue to develop their compact-apportioned water supply, The plan is not, however, intended to offset the salinity fluctuations that are a result of the River's highly variable annual flows (natural variations in the hydrologic cycle), Analyses have shown that the impact of natural variations in the hydrologic cycle can have a significant impact on salinity, These natural variations in runoff can cause a fluctuation in average annual salinity concentration ofas much as 450 mgIL TDS at Imperial Dam, Recognizing the variability of the river flow, the plan for maintaining the criteria is developed using a long-term mean water supply of 15 million acre-feet. When river flows are at or above the long-term average annual flow, and reservoirs are full, then concentrations are expected to be at or below the numeric criteria, When evaluated using this assumption, the flow-weighted average annual salinity is maintained at all times at or below 19721evels, . In addition to the highly variable annual flow, the frequency, duration, and availability of carryover storage greatly affect the salinity of the lower mainstem, Therefore, it is probable that salinity levels will exceed the numeric criteria in some years and be below the criteria in others, As long as adequate control measures are included in the plan, periodic increases in salinity above the criteria as a result of reservoir conditions or periods of below long-term average annual river flow will also be in compliance-with the standards, Provision for Reviewinl!' and Revising Standards The Forum, in its statement of "Principles and Assumptions for Developmentof Colorado River Salinity Standards and Implementation Plan," approved by the Forum on September 20, 1974, stated, under Principle 7: "The plan of implementation shall be reviewed and modified as appropriate from time to time, but at least once every three years, At the same time, the (numeric) standards, as required by Section 303(c) (I) ofP,L. 92-500 shall be reviewed for the purpose of modifYing and adopting standards consistent with the plan so that the Basin states may continue to develop their compact-apportioned waters while providing the best practicable water quality in the Colorado River Basin," . 3-2