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1 Executive Summary 1 1 1 1 RESERVOIR OPERATIONS STUDIES Overview of Methodology Operations studies of existing reservoirs and of the enlarged Elkhead Reservoir were carried out to assess the water supply capabilities and other hydrologic effects of the proposed project. These studies utilized a computer model of the basin developed as part of the previous basin study, This model simulates the hydrology, water rights administration, and reservoir operations of the basin on a monthly basis over a 1930 through 1982 hydrologic study period. The reservoirs operated explicitly in the model are Stagecoach, Steamboat Lake, and Elkhead reservoirs. These reservoirs are modeled to operate, as closely as possible, according to their existing water rights, storage contracts, and other operating agreements. Stagecoach Reservoir is operated as a municipal, industrial, and irrigation water supply and to generate hydropower and maintain minimum flows in the Yampa River below the dam. Steamboat Lake is held nearly full for recreation purposes and is used only as an infrequent source of supplemental water for the Hayden Station. The existing Elkhead Reservoir is operated solely as a supplemental water supply for the City of Craig and the Craig Station. The basin model is an application of a network flow modeling system. Accordingly, the various inflows, river reaches, conveyance and storage facilities, water rights, and other institutional features of the basin are represented as arcs in a network. This modeling approach permits the determination of monthly flow conditions throughout the basin and of monthly diversions and account balances of water rights and contract pools. Preliminary Flow Recommendations at Maybell One of the first applications of the basin model in the present study was to develop preliminary flow recommendations at Maybell. The U.S. Fish and Wildlife Service ("Service") has defined flow recommendations at Deerlodge Park. However, during the previous basin study it became clear that corresponding flow recommendations were needed at Maybell in order to carry out a preliminary Biological Assessment of the proposed Elkhead enlargement (this separate document was submitted to the Service in September of 1994). However, the modeling approach used by the Service in developing its flow recommendations did not permit the unambiguous identification of these corresponding Maybell recommendations. Accordingly, the basin model was re-configured to incorporate the water demand and depletion assumptions embodied in the Service's modeling effort. These assumptions reflected current basin water demands plus additional assumed depletions at the Hayden and Craig generating stations and at Stagecoach Reservoir. These additional assumed depletions exceed estimated current potential demands by roughly 13,000 of/yr. The resulting median simulated flows at Maybell formed the basis of the preliminary flow recommendations. The recommendations are preliminary because they are still under review by the Service. The recommendations are summarized in Table S-1.. S-2