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. DRAFT 4.1.3 Ice Conditions . Mild conditions during the 1986-87 winter generally prohibited the extensive formation of ice within the study area. with the exception of the Y~a River, which was frozen for the majority of the winter, and a 20-mile reach of the Green River at the lower boundary of the study area, which froze over for a period of about 3 weeks, ice formation was very minimal and temporary in nature. Ice conditions during the 1986-87 winter are described as follows: . . 1. Solid surface ice. Freezing of surface water in the river channel occurred throughout the study area to varying degrees. Thin layers of ice were often noted near shorelines or on backwaters in the morning, but usually melted as the main channel warmed during the day. During extended periods of cold weather this type of ice persisted for days, reaching thicknesses of 1 to 2 inches. Periodically, large flucuations in water level effectively cleared the main channel of this solid surface ice. Surface ice persisted on backwaters for most of the period from mid January through February. . During a period of extremely cold temperatures, extending from January 16 to January 28, solid ice formed across a majority of the river channel from Jensen (RM 302) downstream beyond the lower boundary of the study area at OUray Bridge (RM 248). Large open leads of water were present at many places down to Horseshoe Bend (RM 287). From that point downstream to OUray the river was mostly covered with ice. Few observations were made on these ice conditions since this time period coincided with a period between tracking trips. . Solid ice cover formed on the Yampa River in the Echo Park area (RM 345) sometime between December 5, 1986 and January 12, 1987. Heavy ice flows in the Green River near Split Mtn. (RM 319) on March 9, 1987, indicate that breakup of the Yampa occurred on about March 7th or 8th. During the period between January and March, a solid sheet of ice covered the Yampa River from its confluence with the Green River upstream, except for two small open leads of water along the thalwag. Ice thickness on the Yampa River varied from 0.8 to 1.8 feet depending on time of year and location relative to the current. The thickest ice observed in the lower mile of the Yampa was in a backwater at RM 0.7. A squawfish was found in this backwater during a majority of the winter until after breakup, at which time the fish relocated in the main channel at RM 0.2 miles. . . . The breakup of this solid surface ice was observed twice during the course of the 1986-87 winter. On March 9, 1987, beginning at 1530 hours, a high density of floating ice began to appear in the Green River at RM 312.3. Most of the ice was 1.0 to 1.5 feet thick and ranged in size from 3 to 10 square feet. The sound of ice grinding against ice and the river substrate was quite audible. This disturbance of the substrates by the moving ice noticably increased water turbidity. The flow of ice continued through the day, but by the following day only small amounts of ice were floating in the channel or lodged in shallow areas, and the turbidity of the water had decreased. . 2. Jam river ice. A condition termed jam river ice was also observed during the study. This condition is formed by the fracturing, stacking, and compression of solid surface ice, and was more prevalent in 1987-88 winter. 16 .