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Average dissolved oxygen cone ntration in backwaters varied from 6.0 to 13.6 mgIL in 1993- 94 and from 5.1 to 16.3 mgIL in 199 -95. Most recorded concentrations were at or just below saturation for the given temperature an elevation. Low concentrations of 5.1 and 5.3 mgIL in two backwaters in 1994-95 were attributed t local biological oxygen demand from a dense bottom layer of dead leaves and vegetation, as well animal fecal pellets associated with a nearby beaver pond. The high concentration of 16.3 mg/L as recorded in a backwater that was isolated from the main channel by an ice plug formed at the m uth. A substantial amount ofperiphyton (i.e., green algae, diatoms) was present on the cobble S/libstrate and the ice was clear and absent of snow cover.. suggesting active photosynthesis and dxygen production in the closed system. In summary (Table 7), of 23 b ckwaters observed and measured in the winters of 1993-94 and 1994-95, 12 (52%) became unsui ble as overwinter nursery habitat as a result of ice jams that caused damming and flow-through (26 0), reduction of average water depth to less than 0.3 m (13%) as a result of ice development and coli psing ice lenses, or unexplained local channel changes that caused flow-through (13%). Water hemistry appeared to remain suitable over winter for all backwaters measured with a minimum water temperature greater than -0.5 oC and dissolved oxygen concentration greater than 5.0 mg/L. A so, collapsing ice lenses filled parts of back waters, reducing a\'ailable free water and depth, and cr ating ice plugs at the mouths of some backwaters, possibly impeding movement offish to and fro the main channel. ' Table 7. Summary of paramete s observed for experimental winter releases from Flaming Gorge Dam. Parameter Dam releases Air temperature above monthly maxima Air temperature above monthly minima Duration of ice cap Backwater temperature Dissolved oxygen 199 -94 1994-95 low steady 2.9- .0oC 2.9-5.50C 2.9- .90C 3.5-1O.50C 5 d ys 0.0~-3.660C 6.0j13.6 mgIL DISCUSSION 42 days 0.1O-4.780C 5.1-16.3 mg/L Since shortly after Flaming G rge Dam began impounding the Green River in 1962, flows during winter months have been hig er by an average of over 1,000 cfs, and daily stage changes occur for a considerable distance do stream as a result of hydropower generation. Hypolimnetic releases of 8-140 C have kept the ri ve relatively colder in summer, and releases of 4_90 C, combined 20