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<br />II. WATER SUPPLY AND USES <br /> <br />Natural Surface Supply - The surface water supply of the Upper <br />Colorado River Basin is measured or computed at Lee Ferry, the <br />boundary point between the Upper and Lower Colorado Basins. <br />Available records show an average annual natural flow of 15,0 million <br />acre-feet for the 1906-1973 period. The flow has ranged from a low ' <br />of 5.6 million acre-feet in 1934 to a high of 24.0 million acre-feet <br />in 1917. In addition to the extreme variability from year to year, <br />multiyear periods of persistent below or above average occurred. <br />Use of water within the Upper Basin is highly dependent upon <br />storage facilities and the laws and compacts that govern the Colorado <br />River. Only a portion of the 15.0 million acre-feet is available for <br />use in the Upper Colorado River Basin. Storage facilities are <br />needed for most projects and functional water uses throughout the <br />basin for annual regulation and short-term carryover. The major <br />main stem Upper Basin reservoirs, Flaming Gorge, Lake Powell, and <br />the Curecanti system, also provide for long-term carryover storage <br />to deliver water to the Lower Basin under terms of the Colorado <br />River Compact of 1922 and to allow continuous use above Lee Ferry. <br />The reliable supply must be further reduced to carry through an <br />extended period of below average runoff with reasonable shortages. <br /> <br />Many attempts have been made to determine the amount of water <br />available to the Upper Basin States for their annual consumptive <br />use. Differing amounts have resulted from varying interpretations <br />of Compact provisions and methods by which studies were made. <br />Two of the most familiar are: <br /> <br />1. Department of Interior - This study has been used to <br />support actions and proposed plans of development such as the <br />Central Arizona Project. It estimated that at least 5.8 <br />million acre-feet would be available for consumptive use <br />annually in the Upper Basin. Pertinent bases and hypotheses <br />used to derive the 5.8 million acre-foot figure include <br />releases to the Lower Basin of 8.25 million acre-feet for <br />power generation and other purposes, operation of the storage <br />p~oject system of reservoirs through the most critical low-flow <br />period of record (1931-1964), remaining capacity of the <br />reservoir system after sediment accumulation through year 2030, <br />utilization of bank storage for a portion of water delivered, <br />evaporation from main stern reservoirs consistent with the <br />other assumptions, and allowance for shortages to irrigation <br />users during subnormal years. <br /> <br />It must be recognized that the determination of the 5.8 million <br />acre-feet as a possible limit of Upper Basin use is only valid <br /> <br />11 <br />