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~\J Flaming Gorge Environmental Impact Statement Hydrologic Modeling Study Report Results of Action and No Action Alternative Analysis R. Clayton and A. Gilmore October I, 2001 Introduction A model of the Green River Basin has been developed to simulate the operations of Flaming Gorge Dam under varying hydrologic conditions. The Green River model was developed for the purpose of characterizing the hydrologic effects to Flaming Gorge Reservoir and the Green River below Flaming Gorge Dam caused by the implementation of the proposed alternatives for the Flaming Gorge Dam Environmental Impact Statement (Flaming Gorge EIS). Two alternatives have been proposed for the Flaming Gorge EIS. The Action Alternative requires Flaming Gorge Dam to be operated to achieve the flow recommendations described in the "Flow and Temperature Recommendations for Endangered Fishes in the Green River Downstream of Flaming Oorge Dam" (Flow Recommendations). The No Action Alternative requires Flaming Gorge Dam to be operated to achieve the recommended flows described in the 1992 Biological Opinion on the Operation of Flaming Gorge Dam (1992 Biological Opinion). At the present time, and since 1992, Flaming Gorge Dam has been operated to achieve the flow objectives of the 1992 Biological Opinion, For each of these alternatives, the authorizing pwposes of Flaming Gorge Reservoir are to be "maintained" in such a way that impacts to these resources are minimized. In the Green River model, rules to operate Flaming Gorge Dam to achieve the flow objectives described in the Flow Recommendations and the 1992 Biological Opinion were developed. These rules were then modified to reduce the occurrences and magnitudes of bypass releases while still achieving the flow objectives of the alternative. Reducing the occurrences and magnitudes of bypasses was the method used to "maintain" the purposes for which Flaming Gorge Dam was authorized. The purpose of this report is to summarize the hydrologic effects observed in the model output as a result of achieving the flow objectives of each proposed alternative, The results in this report focus on the model output from Flaming Gorge Reservoir and the Green River below Flaming Gorge Dam. Modeling Scope The Green River model was created from an existing model called the Colorado River Simulation System (CRSS). The CRSS has been used for several years to identify impacts to reservoirs in the Colorado River Basin under different hydrologic scenarios. Most recently, CRSS was used to quantify the impacts of the proposed alternatives for the Colorado River Interim Surplus Criteria Environmental Impact Study (2000),