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t . . . ., ~ ( j 20 reproductively isolate the species from its common relative, the roundtail chub. Several fish biologists believe that humpback and roundtail chub hybridize when flows are no longer sufficient to isolate the species during spawning (Valdez and Clemmer, 1982). Because of the deep and narrow reaches that typify humpback chub habitat at Black Rocks and Westwater Canyon, the Service believes that major flow changes would be necessary to reduce habitat to the point of jeopardy. It is expected that the conservation measures included in this opinion to protect the Colorado squawfish will also avoid adverse impacts to the humpback chub. For this reason, the Service has determined that the action is not likely to jeopardize the continued existence of this species. BONYTAIL CHUB Little is known about the biological requirements of the bony tail chub, as the species greatly declined in numbers in the upper basin shortly after 1960. Until recently, the Service considered the species extirpated from the upper basin; however, a recently collected specimen at Black Rocks in 1984 exhibits many bony tail characteristics could indicate a small, extant population. It is assumed that, should this species persist in the Colorado River, the preferred habitat would be larger reaches in the Colorado River, downstream from the area of immediate influence of these water sales. The conservation measures included in this opinion for Colorado squawfish will also avoid impacts to the bony tail chub. Therefore, the Service has determined that the effects of water withdrawals due to this Federal action is not likely to jeopardize the continued existence of the bony tail chub. EFFECTS OF THE ACTION AND CUMULATIVE EFFECTS METHODS To determine the effects of the proposed action as well as cumulative effects, an analysis was conducted in accordance with the final rule implementing Section 7 of the Endangered Species Act (51 FR 19926). Five levels of development for the Colorado River were simulated using a computer-hased hydrologic model. The model used was HYOROSS., an accounting-type model developed by Reclamation in Billings, Montana. The levels of water development used in the analysis are as enumerated below: 1. Historic conditions 2. Existing conditions 3. Environmental baseline 4. Environmental baseline with Ruedi Round II/Green Mountain water sales The historic level of development was simulated by operating HYDROSS with U.S. Geological Survey (Survey) gauge data substituted for the HYDROSS natural flow data base at Cameo on the Colorado River and near Grand Junction on the Gunnison River. HYDROSS was then operated to reflect historic diversions during the period 1952-1982.