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I. , .1 I I I I l .1 ! .. I . 'j I .- j I . .. .. e -, e higher in winter; 2) turbidity is decreased; 3) seasonal extremes in water flow are reduced; and 4) dissolved oxygen concentration is reduced immediately below the dam. These environmental changes may cause a major alteration in fish species composition as has been reported in tail-water areas of large dams by Moffett (1949), Pfitzer (1963), and others. In September 1962, just prior to the closure of Flaming Gorge Dam, the Green River and its tributaries from Pinedale, Wyoming, to a point 7 miles above the damsite, were treated with rotenone to eradicate the non-game fish population preparatory to the establishment of a sport fishery in the new Flaming Gorge Reservoir and its tailwaters (Binns, et ai., 1964). Following closure of the dam in November 1962, approxi- mately 90 miles of the Green River have been inundated, and operation of Flaming Gorge Reservoir has resulted in major changes in flow and temperature patterns in the river below the dam. This investigation was a part of a follow-up study to detennine the effects of the fish-control program upon fish populations in Dinosaur National Monument, 46 miles downstream from Flaming Gorge Dam, and to assess changes on habitat and populations in the Monument brought about by the closing of Flaming Gorge Dam. The objectives of this specific phase of the follow-up study were: 1) to describe changes in river environment associated with the closure of Flaming Gorge Dam, 2) to detennine species composition, distri- bution, and abundance of fishes in the Green River from Flaming Gorge Dam to Ouray, Utah, after the closure of the dam, and 3) to compare 196~1966 distribution of fishes with that reported in pre- impoundment collections. STUDY AREA.-The Green River, the largest tributary of the Colo- rado River, originates on the western slope of the Wind River Range near the Continental Divide in western Wyoming. It flows southward across a desert plateau into Utah and enters the deep canyons of the eastern Uinta Mountains. After passing into Colorado for a short distance it re-enters Utah and joins the Colorado River in southeastern Utah approximately 730 miles from its source. The drainage area consists of nearly 45,000 square miles of mostly arid or semi-arid land. The study area extended from Flaming Gorge Dam downstream to the mouth of the White River near Ouray, Utah (Fig. 1). Several tributaries enter the Green River in this 160-mile stretch, but most are intennittent streams which flow mainly during the spring run-off period. The character of the river environment varies considerably in the study area. In Browns Park (between Bridgeport and Lodore) and 298 ~ +- , ,