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. . Storage vs Yield Approaches September 25, 1990 Page 2 Provision of Navajo Reservoir Yield The concept of periodically making 300,000 af of yield available from Navajo Reservoir to assist in the recovery of the endangered fish evolved from the U.S. Bureau of Reclamation (USBR). The thinking was that the endangered fish surveys performed in 1987-1989 seemed to indicate that 1987 had the most favorable conditions (based on capture rate) for the endangered fish. The USBR estimated that a slug release of 300,000 af under future Navajo Reservoir operation would be required to create a June flow similar to that experienced at Bluff, Utah in 1987. The USBR concept also was that the US Fish and Wildlife Service should accept the fact that full time reproduction of the endangered species was unreasonable and that the San Juan should be allowed to provide part-time reproduction needs. Therefore, the USBR investigated the frequency in which 300,000 af of Navajo Reservoir water could be made available during June for the endangered fish. To establish this frequency, the USBR San Juan River model was operated with logic that if Navaio Reservoir contents were at a 2iven level, then the 300,000 af SlU2 release could be made. The level of Navajo Reservoir contents was increased until no shortages were experienced by modeled New Mexico demands. The model determined that even with full New Mexico development of its compact entitlement that 300,000 af of yield could be made available from Navajo Reservoir in four out of every ten years (on the average). As with the storage approach, there would have to41rovisions made that releases for fish or power flows would not be made without the consent of the US Fish and Wildlife Service or the Recovery Plan. This provision is necessary to assure that water would be available for the endangered fish. If this form of a yield approach is adopted as a reasonable and prudent alternative, then New Mexico should expect that whenever there is a certain amount of storage in Navajo Reservoir, perhaps 1.6 million af, then the Recovery Plan would have the option of exercising its call for a release of up to 300,000 af of water to supplement peak flows of the San Juan River. New Mexico would have little control over the timing or volume of this release. Benefits to Endangered Fish A primary difference between the yield and storage approaches to providing water for the endangered fish is the flexibility of providing water when the fish may need it. The yield approach makes water available to the fish when hydrological conditions permit when the fish mayor may not need the water. The estimation that 300,000 ~ Leonard Rice Coosulling Wale! Engineers. Inc.