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(~'i~ ~, .; .., ,. ,. Yampa River Spawning Habitat Investigtions Introduction• Biological studies by FWS, Colorado Division of Wildlife, and others have demonstrated the importance of the lower Yampa River to Colorado squawfish (Ptychocheilus lucius> for spawning. It has been further demonstrated, through radio-telemetry, that Colorado squawfish may travel 100 miles or more downstream from the upper Yampa and upstream from the Green River to spawn in a well defined reach within the lower 20 miles of the Yampa River. Spawning activity, first documented in 1981 and again in 1982 and 1983 (Table 1) appears to be keyed to a reach that extends from Warm Springs rapid at river mile (RM) 4.5 upstream approximately 15 miles to the vicinity of Harding Hole. It further appears, through repeated captures of ripe fish, that spawning activity (i.e. the actual deposition and fertilization of eggs) is concentrated in a unique reach between RM 15 and 19. This segment of the Yampa River is dramatically different from any other throughout its course. It meanders, becomes braided with islands, cobble and sand bars and slows considerably. Large deep pools intermingled with riffles and ,runs coupled with an embricated cobble substrate predominate in this unique river reach. While it has been well documented that Colorado squawfish enter the lower Yampa during their spawning period, little was known, prior to this year, about the physical parameters such as depths, velocities and substrates utilized by the fish during spawning activities. Through application of the Incremental Stream Flow Methodology (ISFM), developed by the Instream Flow and Aquatic Systems Group, FWS, quantification of habitat availability for Colorado squawfish spawning in the lower Yampa River at a variety of flow levels was quantified. Physical habitat data, for use with the ISFM (i.e. depth, velocity and substrate information) were collected on the Yampa River during the 1983 field season at two sites (Figures 1 and 2). The "spawning bar" site at RM 16.5 was selected because reproductive behavior had been observed there in the previous two years. The second; or "replicate site" at RM 18.5 was selected because sexually mature fish had been .collected and radio-tagged fish monitored there in 1981, it exhibited similar characteristics to the site at RM 16.5, and was within the unique reach between RM 15 and 19. Physical data collected at the two sites, and Colorado squawfish spawning utilization curves, developed from CRFP observation data (Appendix 1), were combined with physical habitat simulation (PHABSIM) computer models, developed for use with the ISFM, to determine habitat availability at the two sites for two phases of spawning activity (staging/resting and deposition/fertilization).