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Ice formation in Winter 1 was minimal and temporary, except for the lower 20 miles (RM 270-250) of the study area, where surface ice persisted for about 3 weeks, from mid to late January, 1987. This surface ice thickened to 3 to 5 cm, and was broken and transported downstream by large mainstem fluctuations. Otherwise, surface ice persisted on backwaters and along some shorelines from mid-January through February, 1987. These thin ice layers extended from shorelines and backwaters in the morning, and usually melted as the main channel warmed during the day. Jam ice was observed on the Green River for only a short period in Winter 1, between mid-January and mid- February. Frazil ice was also rare in Winter 1, forming at night during low temperatures, and melting from daytime solar radiation. Although the Green River did not freeze above Jensen during Winter 1, the Yampa River was frozen much of the winter. Breakup of the Yampa River contributed to heavy ice tlows in the Green River on March 9-10, 1987. Winter 2 (1987-88)..Lower air temperatures and releases from Flaming Gorge Dam in Winter 2 produced lower water temperatures and more extensive ice formation than in Winter 1. Monthly maximum air temperatures for January and February were 4.2 to 2.1°C below normal, while minimum temperatures were 3.9 to 2.0°C below normal. Water temperatures averaged 0.0 to 1.7°C from December through February (Figure 7), which caused supercooling of the river to -1.5°C. River ice formation was extensive in Winter 2. An aerial raptor survey in late January by UDWR (personal communication with Miles Moretti, UDWR, April 20, 1988} revealed that the Green River and Colorado River were ice covered to Lake Powell, except for ice-free areas in large rapids in Desolation and Cataract canyons. Diverse ice conditions and formations in the study area appeared to affect local river hydrolo- gry by blocking or diverting normal tlow patterns. These ice conditions were formed by relatively warm dam releases, distance downstream from the dam, and channel gradient. A longitudinal view of the Green River represents gradient and observed ice formation in January and February, 1988 (Figure 8). The river lrom Flaming Gorge Dam to the Ouray Bridge is about 162 miles (260 km) long, with an elevational change of 905 feet (5,605 to 4,700 feet). In Winter 2, water from Flaming Gorge Dam first became supercooled in Lodore Canyon, about 50 miles (80 km) below the dam. Short sections o1' solid surface ice were present for brief periods at Echo, Island, and Rainbow parks. Surface ice in these areas was frequently broken, dislodged and transported downstream by tluctuating dam releases, leaving ice only along shorelines. The central third of the channel was often frec- ilowing and tilled with Frazil ice during cold periods, and with large masses of jam ice during large llow fluctuations. Frazil ice development was extensive and, at times, tilled the channel and appeared li