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Filial Report 3-35 September 2000 Valdez and Masslich (1989) observed increased movement of adult endangered fishes in portions of Reach 2 during periods of ice breakup in the Green River. They hypothesized that the increased movement could affect the overwinter survival of the fish. Although the degree to which operations of Flaming Gorge Dam caused breakup and movement of ice in the Green River was unclear at the time, the 1992 Biological Opinion (USFWS 1992) recommended stable flows between ice formation and breakup. The reasonable and prudent alternative of the Biological Opinion called for additional studies to evaluate effects of fluctuating flows on conditions in overwintering areas for the endangered fishes. Prior to construction of Flaming Gorge Dam, surface ice probably formed in Reach 1 beginning in December and persisted to at least early March, in a pattern similar to that of the Yampa River. Occasionally, mid-winter rainfall or warm periods may have led to ice breakup, but ice breakup was probably rare during winter. Since construction of Flaming Gorge Dam, water temperatures immediatel y downstream of the dam are higher during winter because of hypolimnetic releases of warmer water (approximately 40C) from the dam, and main channel portions of Reach 1 typically remain ice-free throughout the winter. During periods when the air temperature is colder than the water temperature, the river water cools as it travels downstream. During very cold winters, the reported upstream extent of ice cover on the Green River is in the vicinity of Island and Rainbow Parks, although shoreline ice may occur farther upstream than this. During less severe winters, the upstream extent of ice cover is in the vicinity of the USGS gage near Jensen, Utah. The presence of ice cover has been observed at this location in all years for which reliable observations were made (Valdez 1995; Hayse et al. 2000). Valdez and Cow dell (1999) investigated the formation of ice and conditions in backwater areas of Reach 2 under different operational regimes at Flaming Gorge Dam. Although their study was confounded by mild conditions that occurred during the winters of 1993-1994 (i.e., high-volume, high-fluctuation regime) and 1994-1995 (i.e., low-volume, low-fluctuation regime), results suggested that ice processes in backwaters were largely independent of dam operations. Some frazil ice accumulated in the backwaters, but all of the backwaters studied maintained areas of open water greater than 30 m2 and greater than 9 cm deep. The measured concentration of dissolved oxygen under the backwater ice covers was always greater than 5 mg/L. Hayse et al. (2000) investigated the effects of fluctuating flows on main-channel ice processes in the Green River between the downstream end of Split Mountain (RK 515) and the Ouray, Utah, bridge (RK 399). The overall goal of that study was to assess the influence of the daily release schedule of Flaming Gorge Dam on river-ice processes in the study reach, which is known to be used by overwintering endangered fishes (Valdez and Masslich 1989). Analysis of historic measurements of water and air temperature and ice observations demonstrated that the temperature of water entering the study reach near the Jensen gage and just upstream of the Chew Bridge was often OOC during winter and that daily average air temperatures were consistently below OOC during December, January, and most of February. Ice cover was observed in the study reach during every winter for which reliable records were available. Historic observations indicate that the formation