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2 brates in the Green River Below Flaming Gorge Dam, 1963-1965", prepared by William D: Pearson, and the other was a Ph.D. thesis, "Ecological Studies of Native Green River Fishes Below Flaming Gorge Dam, 1964-1966", prepared by C. David Vanicek. Copies of the two theses accompany this report and contain most of the data utilized in formulation of conclusions. Since the closure of Flaming Gorge Dam in 1962, the ecology of the Green River has changed significantly-from the dam to the mouth of the Yampa Rives, 65 miles downstream. Seasonal fluctuations in flow are reduced, daily fluctuations in flow are increased, water temperatures are lower in summer and higher in winter,-and turbidity is reduced. The flow of the Yampa River has retained its pre-1962 characteristics and moderates the environment of the Green River ;such that all species of fish present in the Green River be- fore impoundment are still present below the mouth of the Yampa. Only four of the nine indigenous species present in the study area were found in the Green River above the mouth of the Yampa in post-impound- ment years. The flannelmouth sucker (Catostomus latipinnis), bluehead sucker (Pantosteus delphinus , Tspeckled dace (Rhinichthys osculus), and Colorado chub Gila robusta) were found in these waters,.and were more abundant when the discharge from the dam was reduced for extended periods and water tempera- tures rose to pre-impoundment levels. During years when discharge was high, water temperatures were below spawning thresholds, and none of the native species were known to have spawned successfully in the Green River above the mouth of the Yampa River. Growth rates of Color?? da o sauawfi =, Ptychocheilus lucius, and Colorado chub were studied in detail, and growth of both species decreased greatly in the years following the closure of Flaming Gorge Dam. Introduced rainbow trout have replaced the native fishes in the cold clear water below Flaming Gorge Dam. Species_ -compQSi.tio of sZxeam_invertebrates h r pnge ,xi4, x4e_first, 40- odd miles below Flaming Gorge Dam. Dominant forms in this area are important trout-food organisms which are adapted to.life in the cold clear tailwaters. Species present downstream from this area are similar to those reported in pre- impoundment years with one exception: a stonefly, Classenia sabulosa, which was found occasionally before impoundment but was not collected in 1964-1965. It _is -found,- ho-we_uer,_ _in__several .other intermountain. waters _., MATERIALS, METHODS, AND PROCEDURES During the 4-year study, 667 fish collections containing 24,040 fish were made: 28 collections in 1963 by the late Dr. D. R. Franklin, and the remainder by C. David Vanicek in 1964-1966 (Table 1). Most of the collections were made with seines and gill nets. Four-hundred-seventy-four bottom samples were taken in 1963-1965. Detailed descriptions of materials, methods, and proc.edures utilized are contained in the appended theses.