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<br />INTRODUCTION <br />Tusher Wash Diversion Dam and the adjoining Thayn Power Plant and Green River Canal have <br />been of concern to endangered fish managers since the early 1980's. The endangered Colorado <br />pikeminnow (Ptychocheilus lucius), razorback sucker (Xyrauchen texanus), and humpback chub <br />(Gila cypha) all inhabit the Green River in the Tusher Wash area (Tyus et al. 1987). Colorado <br />pikeminnow that occupy the lower Green River migrate upstream to the Three Ford Rapids <br />spawning site in June to mid August, and they must negotiate the diversion dam. Larval <br />pikeminnow hatch and disperse downstream past the dam to nursery habitats in the lower river in <br />July, August, and September; at this time five percent of the water is diverted from the river into <br />the Green River Canal system and through the power plant (Cavalli 2000). No doubt some <br />(albeit unquantified) larvae (both native and threatened and endangered) are lost to the raceway <br />and canal system. Adult and sub-adult pikeminnow and other endangered fishes are often <br />attracted into diversion canals and may become stranded when the canals are dewatered at the <br />end of the irrigation season. <br />The Thayn Power Plant's original application to the Federal Energy Regulatory Commission for <br />exemption of small hydroelectric power project from licensing in 1982 was the first report to <br />mention these concerns (Thayn and Thayn 1983). The U. S. Fish and Wildlife Service's <br />Colorado River Fish Project Office at Vernal, Utah, summarized the life history of the Colorado <br />pikeminnow in the vicinity of Tusher Rapid and commented on the likely effect of the proposed <br />hydropower development (Tyus 1984). Tyus (1984) pointed out that the Colorado pikeminnow <br />life history strategy involves long unimpeded migrations, and any blockage of major stream <br />sections could result in serious impact to this species.