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EXECUTIVE SUMMARY Existing biological information together with a comparison of historical and estimated virgin flow scenarios were synthesized following with reference to the Recovery Implementation Program instream flow review to update the U.S. Fish and Wildlife Services'interim flow recommendations for the Yampa River. Biological information on endangered fish abundance, distribution, and spawning activity, existing fish composition and migratory movement, and ecological concepts related to native fish persistence was integrated to provide the best estimate of flows necessary for the recovery and maintenance of the endangered fishes in the Yampa River and the Upper Colorado River Basin. Flow recommendations were based on the premise that natural variability is a major factor in the recruitment and survival of endangered fishes. The major factor affecting the decline of the "large river" endangered fishes in the Upper Colorado River is a failure to recruit. This failure is due to habit~t alteration and the interactions with nonnative fishes in moderated environments. The reproductive biology of Colorado squawfish, razorback sucker and humpback chub is critically linked with the hydrograph. In addition, fishes native to the Colorado River drainage are more abundant in variable environments than nonnative fishes. In this respect, flow recommendations were developed that strive to maintain annual, seasonal and daily variations. Annual variation should be maintained by allowing the recommendation for any given year to be dependent upon the flow magnitude of that year. The recommendation for seasonal variation is unchanged from the U.S. Fish and \Nildlife Services' 1990 interim recommendation that spring flows be whatever occurs during the high flow period of a given year, minus the baseline depletion allowance. Daily flow variation will be achieved through the maintenance of a natural hydrcgraph curve, rather than a truncated, stair-stepped average based on a mean monthly flow. The flow recommendation for the baseflow months (August through March) is a range between 20% and 80% exceedance, dependent upon the magnitude of the flow year. The baseflow recommendation for any specific year would vary between the two ranges based upon the expected and observed flows. In updating the flow recommendations, information gaps associated with biological monitoring for the lower Yampa River and fish passage were identified. Recommendations are made to address those information gaps. v