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RESULTS Ice Development and Breakup Air temperatures in the upper Green River during the winters of 1993-94 and 1994-95 were unusuallywarm. MonthlymaximaatJensen for December through March, 1993-94, were2.9-9.0oC above normal and monthly minima were 2.9-4.90C above normal (Figure 4). Monthly maxima during winterof1994-95 were 2.9-5.5 oC above normal and monthly minima were 3.5-1 0.5 oC above normal. Despite similar above-normal air temperatures during both winters, mainstem water temperatures in the nursery area were substantially different (Figure 5) because of the different volumes of water released from Flaming Gorge Dam. Heat budget, as a product of monthly volume and mean monthly temperature as released at the dam, was 67% higher in 1993-94 than in 1994-95; mean monthly temperature of dam releases was approximately the same in both winters and ranged from about 90C in November to 4 oC in January. Flow and river temperature for winters of 1987-88 to 1994-95 at Green River and Jensen, Utah are presented in Appendix B, as a comparison with preVIOUS years. During winter, 1993-94, mean daily river temperatures near Jensen did not reach freezing (O)C) until early February and temperatures below l.OoC were recorded only 4 days. An ice cap 10nned on the river near Jensen beginning February 2, 1994, but persisted only 5 days. During winter of 1994-95, mean daily river temperatures near Jensen were frequently below 1.0oC, and an ice cap covered the river from mid-December to late January for a period of 42 days. Minimum mainstem temperatures for the three reaches during 1993-94 were similar with a range of -0.05 to 0.32 DC in Reach 1 (Island Park to Rainbow Bridge), -0.10 to 0.380C in Reach 2 (Split Mountain to Bonanza Bridge), and 0.4 to O.soC in Reach 3 (Bonanza Bridge to Ouray). During winter, 1993-94, when special releases were high with high fluctuations, river ice through all three reaches was unstable with shoreline ice in Reaches 1 and 2 frequently collapsing during low flows and multiple layers of shoreline ice forming at various river stages. There were often large amounts of broken ice and frazil ice in the river following cold periods. Frazil ice development was greatest in Reaches 1 and 2, where it was usually mixed in the water column through steep canyon gradients and became suspended on the surface as large floating mats in lower gradients. These mats, termed 'lily-pad ice' (Ashton 1980, 1983), were typically 1-3 m in diameter and 0.3-0.5 m thick, and were the precursor to surface ice or an ice cap. Ice jams were most common in Reach 2, near the Chew Bridge (RK 509) and Bonanza Bridge (RK 467), and in Reach 3, near Leota Bottom (RK 409-411) and Old Charlie Wash (RK 401- 404). These ice jams typically formed when mats of jumbled, broken surface ice and frazil ice became lodged in narrow portions of the channel, temporarily damming the river and diverting flow into high water chute channels. Ice jams within 2 kIn downstream from the Bonanza Bridge increased river stage locally by 1.0-1.3 m during both winters. Ice breakup occurred during February and March of both winters with increasing air and water temperatures. Ice jams during breakup were not as large and persistent as those that formed during colder periods. We note that breakup in the Yampa River released large volumes of water and ice, contributing to breakup of the Green River. 11