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Observed surface ice and mean daily river temperatures and minimum daily air temperatures recorded near Jensen, Utah for winters 1986-87 and 1987-88 (Figure 6) are shown for comparison between the relatively mild winters of this study and a colder winter, such as 1987-88, when surface ice persisted from mid-December to mid-February. Backwater Suitability Of 14 backwaters selected for detailed measurements and observations in 1993-94, 10 were chute channels, 3 were scour channels, and 1 was an eddy return channel (Table 3). Of the 9 backwaters selected in 1994-95, 4 were chute channels, 4 were scour channels, and 1 was an eddy return channel (Table 4); only the .eddy return channel persisted from the previous winter. In 1993- 94. 7 of the 14 backwaters surveyed (50%) were rendered unsuitable as nursery habitat by flow- through caused by ice jams (4), unknown local channel changes (2), or collapsing ice lenses (1). In 1994-95,5 of the 9 backwaters (56%) surveyed became unsuitable because of flow-through caused by ice jams (2), unknown local channel changes (1), collapsing ice lenses (1), and reduced depth from ice thickness (1). Direct observations confirmed that local ice jams partially dammed and elevated the river, causing increased flow through four backwaters in 1993-94, including one scour channels (#3 in Table 3) and three chute channels (#4, #10, #14) that eventually eroded retaining sandbars. Thi~ erosion transformed these backwaters into flow-through channels, negating their value as nursery backwaters. A similar phenomenon was seen in 1994-95, when 2 of the 9 backwaters observed were also affected by ice jams; an eddy-return channel (#4 in Table 4) and a chute channel (#6). Hence, 29% and 22% of backwaters observed in 1993~94 and 1994-95, respectively, were reshaped by ice jams, transforming these sheltered habitats into flow-through channels. This phenomenon occurred in all three backwater formation classifications, i.e., chute, scour, and eddy-return channels. Local channel changes from unknown causes also transformed 2 of 14 backwaters (14%; # 1, # 12) in 1993-94 and 1 of 9 backwaters (11 %; #2) in 1994-95 into flow-through channels. Monthly photographs of sand bars and backwaters in Island Park in winter of 1986-87 and 1987-88 (Valdez and Masslich 1989) showed sand bars migrating and shifting substantially over a 4-month winter period, and hence reshaping backwater habitats. This phenomenon may explain these local channel changes. or perhaps unobserved ice jams occurred between observation periods. In addition, collapsing ice lenses reduced available water depth in one backwater in 1993-94 (#5) as a result of ri\'er stage changes from dam operations and two in 1994-95 (#5, #6) as a result of river stage changes from local ice jams. All backwaters surveyed in both years were ice-covered for much of the winter, despite the absence of a complete ice cap over the mainstem Green River. In 1993-94, average ice thickness on backwaters varied from 2 to 43 cm, and in 1994-95, average ice thickness varied from 3 to 40 cm (Tables 5 and 6). Ice development differed longitudinally and gave characteristic features to the backwaters within each of the three reaches. Detailed descriptions and maps of each backwater are provided in Appendix A. 14