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have on Green River ice processes, the model was used to examine hydraulic conditions throughout the study reach for two alternate release schedules. In the first schedule, the releases were held constant for a number of days. In the second schedule, the releases were varied each day in a manner consistent with a typical peaking pattern used to follow the demand for hydropower. The complete ice model was applied to the Green River study reach using the meteorological and hydrological conditions that occurred during the winters of 1989-1990 through 1995-1996. These are the winters for which both 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. 2.4.1 Unsteady Flow Sub-Model The basis for the Green River unsteady flow sub-model was the UNET one-dimensional unsteady flow model (U. S. Army 1995), calibrated to steady flow data from the Green River Flooded Bottomlands Investigation (FLO Engineering, Inc. 1996) and the observed stage hydrographs collected during the January 25-29, 1997 peaking period. The UNET model simulates unsteady flow in a river channel through solution of one-dimensional continuity and momentum equations. The equations are solved using the four-point, implicit, finite difference scheme. Surveyed and estimated river cross sections, as described below, were used as input to the model to represent the river channel in the study reach. The model time step can be adjusted by the user; a 30-minute time step was used for the results presented in this report. The UNET model can also simulate a floating, stationary ice cover with known thickness and roughness. The composite roughness of the river channel was determined by combining the roughness of the channel bed and the ice cover using the method of Sabaneev (Ashton 1986). The model also accounted for the cross-sectional area of the flow blocked by the ice and the reduction in the hydraulic radius caused by the increase in wetted perimeter due to the ice cover. A number of different boundary conditions can be set by the user for the upstream and downstream limits of the channel. In the present case, a known time-varying discharge was proscribed at the upstream end of the channel and normal depth was set at the downstream end. The upstream boundary of the model used the observed discharge hydrograph at the USGS Jensen gage for 25-30 January 1997. Because no surveyed channel cross-section data were available for the upstream end of the study reach, the channel geometry at RM 316.6 was estimated to reproduce the stage-discharge curve for the Jensen gage (Figure 4). The model used a Manning equation to calculate the normal depth at the downstream-most cross section (located 2 miles downstream from the Ouray Bridge at RM 246). A bed slope of 0.0002 and a Manning's n of 0.035 produced the stage-discharge relationship shown in Figure 5 for the Ouray Bridge location (RM 248). The observed stage of 4654.4 ft MSL at a discharge of 15,500 cfs was from the Green River Flooded Bottomlands Investigation (FLO Engineering, Inc. 1996; see below). -10-