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CHAPTER 1: BACKGROUND and APPROACH Background The Yampa River is the largest tributary in the Upper Colorado River Basin with a relatively natural hydro graph and approximate historical flow magnitude (Tyus and Karp 1989). Yampa River flows are highly variable annually with spring runoff peaks greater, and summer baseflows lower than the Green River into which it empties (USGS records). Self sustaining populations of endangered Colorado pikeminnow Ptychocheilus lucius (Tyus 1990) and one of five known endangered humpback chub Gila cypha populations (Karp and Tyus 1990) reside in the Yampa River. In addition, this river contains one of only two documented spawning sites of both Colorado pikeminnow and razorback sucker Xyrauchen texanus (Tyus and Karp 1990). Because most Colorado pikeminnow spawned in the Yampa River drift downstream to nursery sites in the Uintah Basin, these offspring are important in defining pikeminnow recruitment in the Middle Green, White, and Yampa rivers. Despite, or because of, highly variable environmental conditions, native fishes are more abundant than nonnative fishes in the lower Yampa River (Modde and Smith 1995). Water management recommendations for endangered fishes in the Yampa River began with interim flow recommendations proposed by the U.S. Fish and Wildlife Service in 1990 (USFWS 1990a). The interim recommendation emphasized the need for a natural hydro graph that preserved spring peak flows, but recommended that baseflows not fall below 50% exceedance flow. The 1990 interim recommendation was modified by Modde and Smith (1995) who reiterated that high spring peaks are needed for channel and habitat maintenance, and fish spawning cues, but recommended variable baseflows ranging between 20% and 80% exceedance flows based on the magnitude of the annual hydrograph. Following the modified interim flow recommendations, the Colorado Water Conservation Board filed for a water right to protect the instream flow habitat of endangered fish in the Yampa River (Case numbers 95CWl56 and 95CW 155, filed December 28, 1995). Since these water rights will be junior to existing rights, they will only influence future water development. Existing instream flows in the Yampa River appear to be adequate to maintain stable populations of Colorado pikeminnow and humpback chub (Tyus and Karp 1989). However, in some years, for example 1994, stream flows can be very low (approx. 10 cfs, USGS records). The frequency and duration of low flow periods may increase with continued water development until the state of Colorado develops its full allotment in accordance with the Upper Colorado River Compact agreement. Currently, estimated depletion of water from the Yampa River is approximately 110,000 acre-feet and is forecasted to be as much as 159,000 acre-feet by the year 2040 15