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conducted during the winter of 1996-19971 to make observations of the river ice conditions and to measure the response of the ice cover to Flaming Gorge Dam releases that were steady throughout the day and to releases that fluctuated over a 24-hr period. A flow routing model of the study reach of the Green River was developed using available surveyed river cross-section information. This model, calibrated using the 1996-97 survey data, allowed us to predict and evaluate the effects of daily fluctuations in flow on hydraulic conditions in the study area and to estimate the limits of the river influenced by the fluctuating releases. Finally, a model of river ice processes in the Green River study reach was developed and combined with the unsteady flow model to simulate the formation of ice in the Green River during winters for which appropriate data were available: the winters of 1989-1990 through 1995-1996. Impacts of hydropower releases on ice processes in the Green River were evaluated using these models. 1.1 RIVER ICE FORMATION The formation of river ice cover reflects the meteorologic and hydrologic conditions of the region through which the river flows and the hydraulic conditions of the river channel itself. Ice production in a river begins when the river water reaches a temperature of 0 °C. The river water temperature represents the balance of heat transfer into and out of the river. In most rivers, the dominant heat exchange is between the water surface and the atmosphere. The reservoir behind Flaming Gorge Dam represents a large source of stored heat during the winter. Measurements indicate that the temperature of water released from Flaming Gorge Dam rarely falls below about 4°C in the winter (although Valdez and Masslich [1989] reported that the temperature of releases varied from 1.7°C to 6.2°C during 1987-1988). Downstream of Flaming Gorge Dam, any heat input into the river, other than through the water surface, is probably minor. During cold weather, the Green River cools in response to heat loss from the water surface to the atmosphere. The distance downstream of Flaming Gorge Dam where water temperature drops to 0 °C depends on the release temperature, the heat transfer rate, and the volume of water being released. The point at which substantial ice formation first occurs in the main channel identifies the approximate downstream extent of the winter temperature influence of releases from Flaming Gorge Dam. Records indicate that the winter water temperature in the Green River is often 0 °C by the time it reaches the upstream end of the study reach (Section 2.1). The type of ice formed in the Green River is controlled by the flow conditions in the channel. In the faster moving reaches, frazil ice will form. Frazil ice is ice particles formed in turbulent, supercooled water (i.e., temperatures slightly below the freezing point). Frazil ice does 'Based upon historic meteorological conditions and reports of ice occurrence, the winter period in this report is considered to extend from November through March. Thus, the winter of 1996-1997 refers to November 1996 through March 1997. -2-