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<br />OJ:n88 <br /> <br />. <br /> <br />. <br /> <br />INTRODUCTION <br /> <br />This Biological Assessment has been prepared on the Elkhead Reservoir Enlargement <br />Project to assess any potential environmental impacts of the proposed project on Federally <br />listed threatened or endangered species, as required under Section 7 of the Endangered Species <br />Act, Public Law 93-205, and subsequent amendments. This assessment is based on data and <br />information that has been col1ected during the Elkhead Enlargement Feasibility Study and <br />during other previous hydrological and biological studies conducted in the Yampa River Basin. <br /> <br />Project Purpose <br /> <br />The primary purposes of the Elkhead Reservoir Enlargement Project are to meet a <br />portion of the projected future water supply needs in the Yampa River basin and to offset the <br />effects of those future depletions on instream flows in those lower reaches of the Yampa River <br />that have been designated as critical habitat for endangered Colorado River fishes. An <br />additional important purpose of the project is to provide enhanced reservoir recreational <br />opportunities in Moffat County. <br /> <br />The project will operate to provide supplemental water directly and by exchange to users <br />with inadequate direct flow supplies and to replace certain streamflow depletions that result <br />from diversions under water rights junior to the Juniper Project water rights. These junior <br />diversions benefit from subordinations of the Juniper rights and will cause reductions in <br />streamflows in the lower Yampa River even in the presence of a Juniper-based instream flow <br />right in that reach. The objective of the operation of the enlarged Elkhead Reservoir.is to <br />replace those depletions by junior rights and maintain the instream flow regime that would <br />otherwise obtain from a Juniper-based instream flow water right on the lower Yampa River. <br /> <br />Is is believed that this maintenance of the flow regime in the lower river will enhance the <br />prospects for recovery of the endangered fishes while still permitting moderate levels of in- <br />basin water development. <br /> <br />PROJECT DESCRIPTION <br /> <br />Existing Elkhead Dam and Reservoir <br /> <br />Physical Facilities <br /> <br />Elkhead Dam and Reservoir are located on Elkhead Creek, approximately?? miles <br />upstream of the confluence with the Yampa River, in Routt and Moffat counties (Figure 1). <br />The existing Elkhead Dam is 80 feet high and has a crest length of 1,140 feet at elevation <br />6,375 feet. This homogeneous earthfil1 dam was constructed in 1974 by the Colorado Division <br />of Wildlife for raw water supply and recreation. At water surface elevation 6,365 feet Elkhead <br />Reservoir stores 13,700 af of water. <br /> <br />As described in a 1973 report by Engineering Consultants, Inc. (ECI), Elkhead Dam lies <br />in an alluvial val1ey which is about 600 feet wide at the dam axis. Elkhead Creek crosses the , <br />dam axis at the foot of the right abutment. The right abutment consists of a north-south <br />trending ridge which rises steeply from the channel bottom to an elevation of about 6,400 feet, <br /> <br />1 <br /> <br />/'.. '''- <br />