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000335 INTRODUCTION This Biological Assessment has been prepared on the Elkhead Reservoir Enlargement Project to assess any potential environmental impacts of the proposed project on Federally listed threatened or endangered species, as required under Section 7 of the Endangered Species Act, Public Law 93-205, and subsequent amendments. This assessment is based on data and information collected during the Yampa River Basin Alternatives Feasibility Study (CRWCD, 1993), during an ongoing study entitled Yampa River Basin Recommended Development Alternative Detailed Feasibility Study conducted by the Colorado River Water Conservation District and during other previous hydroIogical and biological studies conducted in the Yampa River Basin. Project Purpose The primary purposes of the Elkhead Reservoir Enlargement Project are to meet a portion of the projected future water supply needs in the Yampa River basin and to seasonally offset the effects of those future depletions on a Juniper decree based instream flow water right in those lower reaches of the Yampa River that have been designated as critical habitat for endangered Colorado River fishes, An additional important purpose of the project is to provide enhanced reservoir recreational opportunities in Moffat County. The project will operate to provide supplemental water directly to users with inadequate direct flow supplies below Elkhead Creek. Junior in-basin water uses that benefit from subordinations of the Juniper rights will cause reductions in streamflows in the lower Yampa River even in the presence of a Juniper-based instream flow right in that reach. One of the objectives of the operation of the enlarged Elkhead Reservoir is to augment those reductions in streamflows made by those junior water uses and maintain the instream flow regime that would otherwise result from a Juniper-based instream flow water right on the lower Yampa River. In order to make the augmentation releases there will be some direct project impacts on river hydroIogy. These include increased reservoir evaporation and peakflow period diversions to storage. PROJECT DESCRIPTION Existing Elkhead Dam and Reservoir Physical Facilities Elkhead Dam and Reservoir are located on EIkhead Creek, approximateIy four miles upstream of the confluence with the Yampa River, in Routt and Moffat counties (Figure 1). The existing Elkhead Dam is 80 feet high and has a crest Iength of 1,140 feet at elevation 6,375 feet. This homogeneous earthfill dam was constructed in 1974 by the Colorado Division of Wildlife for raw water supply and recreation. At water surface eIevation 6,365 feet EIkhead Reservoir stores 13,700 af of water. As described in a 1973 report by Engineering ConsuItants, Inc. (ECI), Elkhead Dam lies in an alluvial valley which is about 600 feet wide at the dam axis. EIkhead Creek crosses the dam axis at the foot of the right abutment. The right abutment consists of a north-south 1