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. DRAFT . The resulting ice mass was characterized by highly irregular upper and lower surfaces caused by large pieces of ice being tilted then frozen into place by water seeping through cracks and crevices. This condition was present for only a short time during the 1986-87 winter, following a breakup of surface ice in the Green River on January 28. At that time, jam ice formed across the channel from RM 276.0 to the lower limit of the study area. This condition persisted through mid February with the upper end of the jam being slowly eroded by melting and current action. At some time during mid February, the channel opened and all the jam ice was flushed down river, leaving only piles of broken ice along the shoreline and sandbars. . . 3. Frazil ice. During periods of extremely low temperatures, supercooling of river water resulted in the formation of frazil ice. Depending on the duration and severity of the low temperatures, the formation of frazil ice ranged from only a few crystals floating in the water to a heavy slush condition. The river often appeared milky and became more viscous from the large amount of ice crystals present. . Varying degrees of frazil ice formation were observed in late December and January of the 1986-87 winter. It was most commonly seen in the morning following subzero nightime temperatures. Most frazil ice was melted by 1000 or 1100 hours as water temperatures were moderated by solar radiation. On January 15 heavy frazil ice formed from Jensen to the lower limit of the study area. This coincided with the onset of very cold temperatures and the subsequent formation of solid ice in the same area on January 16. Frazil ice is often the precursor to solid ice. . 4.2 THE WINTER OF 1987-88 4.2.1 River Flow . Flow regimes during the second winter were generally lower and more variable than the first (Figure 2). The variability in discharge observed during the second winter of the study was related in part, to test flows delivered for two fisheries studies being conducted throughout the winter below the dam; this study as well as a second on the salmonid fishery below the tailrace. . 4.2.2 Air and Water Temperatue . Significantly lower ambient temperatures (Table 2) and lower releases from Flaming Gorge Dam resulted in much lower water temperatures during most of the 1987-88 winter. Monthly means, minimum, and maximum air temperatures during January and February were 3.6 to 7.60 C below normal. This resulted in frequent supercooled conditions causing extensive frazil ice formation in the upper region of the study area. Consistent ice cover from RM 316 to the lower end of the study area served to moderate water temperatures in the lower region so that the water flowing under the ice remained at zero degrees celcius until spring break-up. . 17 .