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MODELING DYNAMIC ICE FORMATION IN THE GREEN RIVER ' The formation and trans ort of river ice and the formation of stationary river ice covers can be P simulated through the use of numerical models (see for example Lal and Shen, 1993; Shen et. al., 1991; Beltaos, 1995). Such a numerical ice model (Daly, in prep.) was applied to the study reach ' of the Green River. This model is composed of aone-dimensional unsteady flow sub-model, a transport sub-model, and an ice cover progression sub-model. The one-dimensional unsteady flow sub-model is the UNET model described above. The UNET model simulates unsteady flow in a river channel by solving the complete one- dimensional continuity and momentum equations. The equations are solved using the four-point, implicit, finite difference scheme and the model used the actual surveyed and estimated river cross sections, as described above, to represent the river channel. In this case, the UNET model has been modified to incorporate the dynamic formation of river ice covers as determined by the ice cover progression sub-model. The ice cover progression sub-model calculates the sections of the channel in which a stationary floating ice cover will form. The presence of a stationary ice cover in a reach changes the hydraulic properties of the reach. These changes include reducing the cross-sectional area of the channel available for flow, reducing the hydraulic radius of the channel cross section, and modifying the effective channel roughness. These changes in the ' hydraulic properties in turn influence the discharge and stage calculated by the unsteady flow model. The transport sub-model calculates the advection of water temperature, surface ice, and suspended frazil ice. Frazil ice production is assumed to begin through the introduction of seed crystals at the water surface. The concentration of the frazil ice is calculated by balancing the heat loss from the water surface and latent heat released from the growing frazil. The frazil ice is assumed to rise to the water surface with a known velocity. At the water surface, the frazil ice forms into floes which are advected by the flaw velocity. The heat loss from the water surface is , calculated as a linear function of the difference between the water temperature and the air temperature. The transport sub-model uses the Preissman-Holly advection scheme (Gunge, Holly, and Verwey 1980). This scheme has been shown to minimize numerical diffusion. The ice cover progression sub-model determines the rate at which stationary ice covers form. A stationary ice cover is assumed to initially form at apre-selected bridging location when the concentration of surface ice has reached apre-selected value. The ice cover then progresses upstream at a rate determined by the rate of arrival of the surface ice and the thickness of the ice cover. The ice cover can thicken through heat transfer to the atmosphere from the ice surface and through the deposition of frazil ice underneath the ice cover. The ice cover can melt out through heat transfer from the water flowing beneath it. When the ice cover has lost a certain percentage of its thickness, it is assumed to collapse and be transported in the downstream direction. The ice model was applied to the Green River study reach for the winter of 1989-90 through the winter of 1995-96. These are the winters for which water temperature and discharge data were available at the Jensen Gage. The daily average discharge, air temperature, and water temperature were used as inputs to the model. The river cross sections and channel roughnesses ' 18