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EXECUTIVE SUMMARY i Non-Native fish have been identified as an impediment to recovery of endangered fish in the upper Colorado basin, the impediment being habitat competition and predation. Control of non-native species has been identified in the recovery action plan as one item that needs to be accomplished to move toward recovery. This report is a feasibility evaluation of installing control structures to eliminate escapement from off-stream reservoirs in the upper Colorado basin. The reservoirs evaluated in this study are Elkhead Reservoir near Craig, Colorado, and Highline Reservoir near Loma, Colorado. This study also serves as a source of reference information and an example of how control structures might be implanted at other similar facilities throughout the upper basin. Objectives of the report were to examine two different levels of control. The first objective examined full exclusion of egg and larger life stages at Elkhead and Highline Reservoirs. The exclusion facility would handle flows up to the 100 year flood event with as close as possible to 100% efficiency. The second objective was to evaluate installing a control structure that limited escapement to the current industry practice. That practice is approximately 90% effectiveness and screen openings no smaller than 3132 inch. Also, the facility would handle flows up to the 100 year flood event. For frame of reference, a no action alternative was also looked at to provide a basis for change in fish community and a perspective on effectiveness of the full exclusion and current industry practice alternatives. Elkhead Reservoir is located on Elkhead Creek which is tributary to the Yampa River. Elkhead Creek has routed flows up to 2,100 cfs and is an unregulated watershed. The reservoir spills annually and is rarely drawn down for release of water for municipal or irrigation use. Highline Reservoir is located on Mack Wash which is a tributary to the Colorado River. Highline receives all inflow, except for local storm runoff, as administrative spills from the Highline diversion canal. Flows into the reservoir and out of the reservoir generally are less than 100 cfs. The watershed upstream of Highline Reservoir on Mack Wash is small and the high flows would be the result of storm runoff, especially summer thunderstorms. Target species evaluated for this control structure feasibility study included channel catfish, smallmouth bass and northern pike. These species are present in both reservoirs with the exception 1 of northern pike which is present only in Elkhead Reservoir. Currently, smallmouth bass reproduce in the reservoirs. Channel catfish are suspected of reproducing only in Highline Reservoir. With /t these species, the life stages of interest include everything from egg size and larger. Other warm ?l water species present in the reservoirs include black crappie and largemouth bass. These species can escape but are not likely to successfully reproduce and recruit to the riverine fish populations. Therefore, these latter species were not the primary focus of the evaluation. Control techniques selected for the target species also should control the non-target species. Evaluations started with a literature review and personal interviews on various screening and fish passage facilities. The interviews were with experts in the field of fish passage, specifically related to design, construction and operation of mostly downstream migrant facilities. Structures evaluated in the present study did not include a fish bypass. The structural option designs were configured so that there would be no bypass of non-native fish downstream and result in handling of those fish for either disposal or transport back upstream to the reservoir. This is a substantial variation from the Control Structure Feasibility Evaluation Miller Ecological Consultants, Inc., February 18, 1997