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<br />Natural Variations <br /> <br />o <br />N <br />~ The standards require that a plan be developed which will maintain the flow-weighted <br />l"\) average annual salinity at or below the 1972 levels while the Basin states continue to develop their <br />compact-apportioned water supply. The plan is not, however, intended to offset the salinity <br />fluctuations that are a result of the River's highly variable annual flows (natural variations in the <br />hydrologic cycle). Analyses have shown that the impact of natural variations in the hydrologic cycle <br />can have a significant impact on salinity. These natural variations in runoff can cause a fluctuation <br />in average annual salinity concentration of as much as 450 mg/L TDS at Imperial Dam. Recognizing <br />the variability of the river, the plan for maintaining the criteria is developed using a long-term mean <br />water supply of 15 maf. When River flows are at or above the long-term mean, and reservoirs are <br />full, concentrations are expected and have been observed to be below the numeric criteria. <br />Conversely, when flows are dramatically below the long-term mean, and reservoirs are depleted, <br />salinities may increase above the numeric criteria. <br /> <br />Temporary Increases <br /> <br />The federal regulations provide for temporary increases above the 1972 levels if sufficient <br />control measures are included in the Plan of Implementation. Should additional water development <br />projects take place beyond those anticipated to occur before control measures are brought on line, <br />temporary increases above the numeric criteria could result. However, these increases will be <br />deemed to conform with the standards if appropriate salinity control measures are included in the <br />plan. During the next three years, or the period of this review, no increases above the 1972 levels <br />are anticipated. <br /> <br />Provision for Reviewing and Revising Standards <br /> <br />The Forum, in its statement of "Principles and Assumptions for Development of Colorado <br />River Salinity Standards and Implementation Plan," approved by the Forum on September 20, 1974, <br />stated, under Principle 7: <br /> <br />"The Plan of Implementation shall be reviewed and modified as appropriate from <br />time to time, but at least once every three years. At the same time, the (numeric) <br />standards, as required by Section 303 (c) (1) ofP.L. 92-5(K) shall be reviewed for the <br />purpose of modifYing and adopting standards consistent with the plan so that the <br />Basin states may continue to develop their compact-apportioned waters while <br />providing the best practicable water quality in the Colorado River Basin." <br /> <br />3-2 <br />