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1 Executive Summary Site Selection Recommendations Based on these evaluations, the multi-disciplinary evaluation team developed a set of preliminary recommendations regarding the sites that should be carried forward into formulation of alternatives. Upon reviewing these recommendations, TSG determined that an ' additional long-term upper basin alternative should be considered, namely, the enlargement of the existing Stagecoach Reservoir. Substantial data on this alternative already existed in the Final Environmental Impact Statement (EIS) for the Stagecoach Project. This permitted the site to be added and considered without conducting additional field evaluation. The sites ultimately recommended for further consideration are listed in Table S-3. Table S-3 Sites Recommended for Further Consideration 1. Stagecoach Reservoir Enlargement 2. Elk Creek Off-channel Storage 3. Elkhead Reservoir Enlargement 4. 5 Williams Fork near Hamilton East Fork Williams Fork above Willow Creek . ' DEVELOPMENT OF BASIN MODEL Once the potential sites for new reservoir storage had been identified, the next objective of the Study was to evaluate each site's potential for meeting future water needs in the basin. A hydrologic computer model of the basin was developed in order to quantitatively perform these evaluations. In addition to simulating the basic hydrologic and water supply operations in the basin, the model permits the inclusion of proposed reservoirs and water management strategies and describes the resulting water deliveries, reservoir levels, and streamflow conditions throughout the basin. The model represents the hydrology, water demands, reservoir operations, and water rights administration of the basin on a monthly basis over a 53-year hydrologic study period encompassing water years 1930 through 1982. This is the same study period used in previous modeling work performed by the Service to develop the Interim Flow Recommendations for the recovery of the endangered fishes in the Yampa River. The period includes a wide range of hydrologic conditions, from drought years to flood years, and permits an assessment of the long-term effects of changes in water development and management. The results of model runs were compared in order to quantitatively determine the ability of different development alternatives to meet basin water needs under a variety of operating assumptions. 1 Model Configuration The Yampa River basin Model was configured as a network of arcs and nodes and ' utilizes network flow programming methods. Figure S-3 shows the basin model network. This network representation of the basin has a physical appearance much like a map of the river including tributaries, reservoirs and diversion points. The model accepts input of gaged inflows and water demands, simulates the allocation of these flows to network arcs ' representing various water rights and uses in the basin, and predicts the resulting streamflow conditions in reaches of interest. The mathematical solution procedure of the model insures S-13