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' endahl January 2000). The upper limit, 26,600 cfs, is based to Smith to Leo Eisel and Sue Upp some extent on downstream flooding considerations and is not totally habitat based. The Service ' has the objective of increasing the frequency of occurrence of flows greater than 12,900 cfs. The Service has identified target flows in the 15-Mile Reach in the range of 12,900 to 26,600 cfs because flows of this magnitude have been shown to mobilize gravel cobble substrate (U.S. Fish and Wildlife Service, December 20, 1999). The Service is not interested in increasing the peak when the actual peak or augmented peak is likely to be less than 12,900 cfs or greater than ' 26,600 cfs. The Service would like to see as much as possible of the average annual 20,000 acre- feet made available during the 7 to 10 day period surrounding the peak spring discharge. ' The frequency and dependability of each alternative for providing the average annual 20,000 acre-feet will be determined by using StateMod with the C1 Scenario as the baseline conditions and with the baseline hydrology as described above. 1 The Service has indicated that the 20,000 acre-feet will be determined as an average annual volume (Smith and Muth, July 1999). In years when flows in the Colorado River at the top of ' the 15-Mile Reach are less than 12,900 cfs or greater than 26,600 cfs, no water will be provided. For those years when the flow is between 12,900 and 26,600 cfs, an average of 20,000 acre-feet ' will be provided, i.e., in some years more than 20,000 acre-feet will be provided, and in some years less. ' With some alternatives it may be possible to carry over unused water to the next year for purposes of meeting the average annual 20,000 acre-feet target in that year. ' If some portion of the average annual 20,000 acre-feet remains after augmentation of the spring peak discharge and this water can not be carried over for use in the succeeding spring, this water may be used for augmenting low flows during the July 15 through October 31 period. ' Augmentation of low flows in late summer and fall is second priority to augmenting the spring peak discharge during the 7 to 10 day spring peak period. ' The following characterize the low flow target: • During the July 15 through October 31 period, the goal will be to maintain minimum ' flows at the head of the 15-Mile Reach as specified by the Service (U.S. Fish and Wildlife Service, May 1995. • During years of above average precipitation (i.e., precipitation greater than the 50th percentile), efforts would be made to maintain flows at or around 1,630 cfs. • During years of low precipitation (i.e., precipitation less than the 50th percentile), efforts would be made to maintain flows at or greater than 1,240 cfs. • During drought years (the lowest 20' percentile), efforts would be made to maintain flows at or greater than 810 cfs. p:\data\gen\0cwcb\I8133\report\phase-1\chap-2.doc 2-9