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. 28 . The land and Water Fund of the Rockies @ Moving Blue Mesa's Marketable Yield: A Myth tion inflow. As a result, it was believed Ihen that the long-term average water supply at the bottom of the Upper Colorado Basin, Lee Ferry, was greater than 15 million acre-feet (MAF) and that there would be at least 7.5 MAF available for use in the Upper Basin on an average annual basis." At the same time, it was estimated that there was at least enough water originating in the Lower Basin to satisfy the Mexican Treaty obligation to deliver 1.5 MAF and satisfy an additional 1.0 MAF of annual con- sumptive use in the Lower Basin. Under these assumptions, Colorado would have as much as 3.8 MAF of annual consumptive use available."u It is now apparent that long-term water supply conditions were greatly overes- timated in 1922. The average annual water supply at Lee Ferry is considerably less than 15 MAF. A new hydrologic determination conducted by the Bureau in 1988 found that there were only 6.0 MAF available to the Upper Basin states."" This esti- mate entitles Colorado to use less than 3.1 MAE In the words of a workgroup that looked closely at the subjecI, Colorado "needs to be aware that it could be taking sig- nificant [legal] risks if it makes extended use of more than 3.079 MAF annually."'" The workgroup referenced above, called the Endangered Fish Flow and Colorado River Compact Water Development Workgroup, was assembled by the Colorado Water Conservation Board ("CWCB"). The CWCB hoped the Workgroup would provide "input on how much water can be appropriated for endangered fish recovery instream flow purposes within the various [Colorado River] sub-basins. . . without impairing Colorado's ability to fully develop its compact apportioned waters.""3 This workgroup determined Colorado's current consumptive uses and remaining, undeveloped entitlement from the Colorado River. 104 Taking data on water use from the years 1981-1985, it found the current average annual consump- tive use of Colorado River water was 2.3 MAF and the maximum use was 2.6 MAF. Thus, considering the lowered entitlement estimate, the range of what Colorado has left to develop runs from a minimum of 450,000 AF to a maximum of 1.5 MAF.'U5 The workgroup then recommended an approach that distributed Colorado's remaining Compact apportionment among the State's tributary basins using the ranges above. After distributing the state's apportionment surplus among the seven major sub-basins according to each sub-basin's contribution to the natural flow of the Colorado River, it estimated future water development capacities for each sub-basin. Based on this scheme, it found the upper limit on future development in the Gunnison to be 225,213 AF and the lower limit to be 99,227,"0 with the remainder of the river's 1.8 MAF annual discharge needed to meet downstream delivery require- ments. Under this approach, even if we assumed away all of the many other uses of Aspinall water that compete for its yield, there is a maximum of 225,213 AF available for consumptive use out of Aspinall's marketable yield. Adopting a conservative and prudent approach, strongly suggested by the drought, would establish a marketable yield far lower. And, of course, there are other uses competing for this water, includ- ing the in-stream uses for endangered fish and Black Canyon National Park, as well as hydropower and future in-basin development.