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BACKGROUND Purpose This document proposes flow recommendations (as measured at the Maybell gage) needed to recover Colorado squawfish Ptychocheilus lucius, humpback chub, Gila cypha, bonytail, G. elegans, and razorback sucker, Xyrauchen texanus, in waters influenced by Yampa River flows. In addition to the four listed species, the Yampa River drainage has some of the last abundant populations of flannelmouth sucker, Catostomus discobolus, and roundtail chub, G. robusta. As the last major, unregulated tributary in the Upper Colorado River Basin, the Yampa River provides flows that support the largest natural populations of Colorado squawfish and razorback sucker in the Colorado River Basin. The approach taken in this document is to use the best data and science available to define the hydrological needs of endangered fishes. It is not the purpose of this document to define the mechanisms to achieve the recommendations proposed, but rather, to use existing knowledge to define water needs for recovering and maintaining habitat for endangered fishes in Yampa and Green rivers. We recognize that sufficient flows to meet the following recommendations and projected water depletions may not always be available and that, at times, limitations will be placed on the implementation of the recommendations during some times of the year. A discussion of these limitations is found in the section that summarizes flow recommendations. The recommendations proposed in this document update the 1990 interim Yampa River flow recommendations presented by the U.S. Fish and Wildlife Service (USFWS 1990). The interim recommendations were based. 011 a review of existing biological data on endangered fishes by Tyus and Karp (1989). That approach was selected following the failure of physical habitat modeling (IFIM) to demonstrate predictive cause-and-effect relationships with the distribution and abundance of endangered fishes in the Green River Basin (Rose and Hahn 1989). The recommendations proposed in this document are heavily based on the 'biological . information- presented by Tyus and Karp (1989), but. also include information provided by the RIP (Recovery Implementation Program for the Recovery of Endangered Fishes in the Upper Colorado River Basin) instream flow review (Stanford 1994),' Comparison of historical and virgin flow estimates of the Yampa River (Maybell), and published ecological principles. .' ' Study Area Originating in the White River National Forest on the western slope of the . Rocky Mountains, the Yampa River is the largest tributary of the Green River (Figure 1). The greatest abundance of endangered native fishes in the Yampa River occurs downstream of (potamon-rithron transitio'n region of the river) Hayden, Colorado. The upper reach of the Yampa River occupied by endangered fishes flows through low gradient agricultural valleys with the exception of Juniper (RM 90.5 - 88.2) and Cross Mountain (RM 58.5 - 55.2) canyons prior to entering the high gradient Yampa Canyon (RM 46.7 - 0). 1