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<") '5~'~ ~A ~) ~4' 7 The period of reservoir filling occurred in 1963-1966. The first full year of "normal" operation began in 1967 and the 1967-1978 period 'represents pre- multilevel outlet hydrology. The 1979-1984 period reflects dam operation after installation of a multilevel outlet structure. Finally, the 1985-1988 period reflects operation during the "interim flow agreement" for protection of critical endangered fishes nursery habitats downstream of Jensen, Utah, and other special releases associated with study objectives contained in the inter-agency agreement (6-AA-40-04070) between the Service and Reclamation. BASIS FOR OPINION The endangered Colorado squawfish, humpback chub, bony tail chub, and razorback sucker inhabit the Green River. Occupied habitat currently extends from the confluence with the Colorado River at river mile zero (0) upstream to near the Willow Creek confluence (Swallow Canyon) at river mile 383.5. The mainstem Green River and its tributary, the Yampa, contain the largest populations of Colorado squawfish and razorback sucker known to exist in natural riverine habitats. Humpback chub have a limited, discontinuous distribution in canyon- bound habitats and persist in small numbers in Desolation and Whirlpool canyons. The bony tail chub is extremely rare throughout the Upper Colorado River Basin. Figure 3, taken from Tyus and Karp (July 1991), shows the conceptual relationships between a natural annual distribution hydrograph and the timing of life history requirements for the Colorado squawfish, humpback chub, and razorback sucker. It is this natural hydrograph with a large spring peak, a gradual descending limb into early summer and low, stable flows through the summer, fall and winter that the Service believes creates the best habitat conditions for rare and endangered fishes in the Green River Basin. This flow scenario maintains long term stability and geomorphology of the channel, prepares spawning areas, and forms nursery habitats. DEVELOPMENT AND EXTENT OF PERTINENT SCIENTIFIC DATA BASE The Endangered Species Act (Act) of 1973 (P.L. 93-205) charged all Federal agencies to consider the needs of species listed as threatened and endangered. The Act also mandated all Federal Agencies to consult with the Service to insure that their actions were not likely to jeopardize listed species. Early consultations revealed the need for scientific information on the endangered species. In 1979, the collection of life history data to expand the scant data base that existed for the rare and endangered fishes of the upper Colorado River Basin began with formation of the Colorado River Fisheries Project. In 1982 Reclamation and the Service continued studies on endangered fishes under the Colorado River Fishery Monitoring Program. These studies were to examine the effects of flows from Flaming Gorge Dam primarily on Colorado squawfish. In 1985 the Service and Reclamation agreed to an interagency Flaming Gorge Studies program that began in 1986 and ended in 1990. That program evaluated habitat requirements and streamflow needs of the rare fishes and provided flow recommendations in a final report dated July 1991 (Tyus and Karp 1991).