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17 I I I I I I I I I I I I I I I I I I I December. By the middle of December, frazil and dislodged shoreline ice had formed several ice jams. One ice jam just below the Chew Bridge had increased river stage and diverted flow into the high water flood plain. Another ice jam near the Bonanza Bridge increased river stage by almost 4.0 ft. After peak ice formation the first week of January, jam and shoreline ice quickly cleared and the main channel was free of ice by the first week of February. In reach 3 river ice conditions were similar to Reach 2~ with ice formation beginning the first week of December, forming large ice jams. As in 1993-94, extensive ice jams formed within the Ouray National Wildlife Refuge (RM 255.5 to 254.0) and another at Old Charlie Wash (RM 251.3 to 249.0). River stage in this area was elevated enough to flood bottom lands. After peak ice formation in January, jam and shoreline ice quickly cleared. The mainchannel was free of ice by the first week of February. 6.0 RESULTS Totals of 14 and 9 backwaters were selected for measurements and observations in winters of 1993- 94 (year 1) and 1994-95 (year 2), respectively. Of the 14 backwaters selected in year 1, 10 were chUte channels, 3 were scour channels, and 1 was an eddy return channel (Table 7). Because of the lower water levels in year 2, only the eddy return channel persisted from year 1 to year 2. Of the 9 backwaters selected in year 2, 4 were chute channels, 4 were scour channels, and 1 was an eddy return channel (Table 8). Five of the 14 backwaters observed in year 1 became flow-through channels between the time of the original measurements in late September, 1993 and the first trip in early January, 1994. Direct observations were made confirming that local ice jams partially darned and elevated the river, causing increased flow through one scour channel (#3) and two chute channels (#4, #10) that eventually eroded the retaining sandbar. This erosion transformed the backwaters into flow-through channels, negating their value as nursery backwaters. A similar phenomenon was seen in year 2, when 2 of the 9 backwaters (#4, #6) observed were similarly affected by ice jams; one was an eddy-return channel and the other was a chute channel. Hence, 21% and 22% of backwaters observed in years 1 and 2, respectively, were reshaped by ice jams, transforming these sheltered habitats into flow-through channels. This phenomenon occurred in all three backwater formation classifications, Le., chute, scour, and eddy-return channels. Natural geomorphic changes, unrelated to ice conditions, also transformed 2 of 14 backwaters (#1, #3) in year 1 and 1 of9 backwaters (#2) in year 2 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, and hence reshaping backwater habitats. Hence, 14% and 11% of backwaters observed in years 1 and 2, respectively, were reshaped by natural geomorphic changes, unrelated to ice conditions. All backwaters observed in both years were ice-covered. In year 1, average ice thickness varied from <0.1 to 1.4 feet, and in year 2, average ice thickness varied from 0.1 to 1.3 feet. Ice formation 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 B. The ice on backwaters in reach 1 (Island Park to Rainbow Park) was typically a continuation of shoreline ice development that was more extensiv~ in the backwaters because of a lack of current. The central