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Minimum physical and chemical habitat requirements were designated and used as criteria to determine if a backwater could support fish through winter. Changes in backwater morphology during winter were not considered detrimental to habitat value as long as minimum habitat requirements were not violated. The primary physical variables of importance to fish in backwaters are believed to be size of backwater and depth of free water. Only backwaters which met the minimum requirements of the ISMP for a suitable backwater were selected for this study (i.e., surface area of at least 30 m2 and maximum depth of at least 0.3 m). The criterion of 0.3 m of free water was used as the minimum depth of water useable by fish, either between ice cover and the river substrate or between ice layers. Free water of less than 0.3 m was not considered suitable habitat because of the constricted space and the likelihood of a supercooling effect from frazil ice (Ashton 1983). which can be lethal to fish (Lagler et al. 1962). Chemical characteristics were measured using a Hydrolab Surveyor 2 (Hydrolab Corp., Austin. Texas ). Water temperature (:l:: 0.01 OC), dissolved oxygen (mg/L), conductivity (,uS/cm), and pH were measured in the main channel adjacent to the backwater, then along the deepest portion of the backwater beginning at the mouth and working toward the back. Typically, four to six measurements were taken within each backwater when free water was present; fewer were taken when ice was present, depending on the extent of the ice cover. Water temperature and dissolved oxygen were the two primary chemical components considered important to overwintering fish. Minimum temperature requirement for backwater habitats was set at -0.50C. No information on dissolved oxygen tolerance limits for Colorado pikeminnow was found, so a low of 5 mgIL, as potentially lethal for Ictalurids(catfishes) and Centrarchids (sunfishes) was used (Whitmore et al. 1960; Moss and Scott 1961; Bonn et al. 1976; Piper et al. 1982; Tucker 1985; and Stickney 1986). HYDROLOGY OF THE GREEN RIVER Special releases from Flaming Gorge Dam during the winter of 1993-94 were of a higher volume with greater daily fluctuations than during the winter of 1994-95 (Table 2). The 1993-94 releases approximated maximum hydropower generation. Mean daily flows at Greendale varied from a high of2,584 cfs in December to 1,125 cfs in March, and monthly volume released varied from 158.900 acre- feet in December to 69,160 acre-feet in March. Hourly releases varied from about 1.000 cfs to nearly 4,000 cfs during the months of December through February (Figure 2). On a daily basis, lowest releases usually occurred between 0100 hours and 0500 hours, and highest releases usually occurred between 1900 hours and 2200 hours (Figure 3). Mean daily flows near Jensen varied from ahighof2,959 cfs in December to 2,401 cfs in February. Despite lower releases ti'om Flaming Gorge Dam in March, flows near Jensen were higher because of the start of spring runoff in the Yampa River, which flows into the Green River 105 km downstream from the dam. Releases in winter of 1994-95 were of a lower volume with less daily fluctuation than in winter of 1993-94. In 1994-95, mean daily flows of the Green River at Greendale varied from a high of 1.305 cfs in December to 828 cfs in March, and monthly volume released varied from 80,250 acre-feet in December to 50,920 acre-feet in March (Table 2). Hourly releases from December through February varied by less than 300 cfs (Figure 2). On a daily basis, hourly releases were nearly constant (Figure 3). Mean daily flows near Jensen varied from ahighofl,993 cfs in March to 1,319 7