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INTRODUCTION Identification of spawning azeas of rare and endangered species is important to recovery efforts. In the upper Colorado River basin (UCRB), such identifications have allowed researchers to strengthen broodstock with genetically diverse wild stocks (Czapla 1998), describe reproductive populations (Growl and Bouwes 1997), and aid in establishing management objectives (Lentsch et al. 1998). The Green River and its tributary the Yampa River hazbor reproducing populations of both razorback sucker and Colorado pikeminnow, at Jensen, Utah and in Yampa Canyon, Yampa River and Gray Canyon respectively. Empirical evidence (Gutermuth 1994; Muth et al. 1998; Tyus et al. 1987; Valdez 1990) suggests that razorback sucker (Xyrauchen texanus) and Colorado pikeminnow (Ptychocheilus lucius) spawn in the lower Green River below the town of Green River (RK 193.2, RM 120.0). In 1994, Utah Division of Wildlife Resources (UDWR) initiated a study to evaluate the possibility of individuals of these two species spawning in the lower Green River. The investigation of razorback sucker reproduction was undertaken in conjunction with studies being conducted by the Larval Fish laboratory (LFL), United States Fish and Wildlife Service (IJSFWS), and National Pazk Service (NPS). Results of portions of this study and the accompanying related investigations have been presented in Muth (1995), Muth and Wick (1997), and Muth et al. (1998). Razorback sucker Razorback sucker is a lazge catostomid chazacterized by a large nuchal keel, which is endemic within the upper Colorado River basin. Listed as a federally endangered species in 1991 (56 Federal Register 54957), the lazgest remaining riverine population is found in the middle Green River near Jensen, Utah (RK 484.6, RM 301.0). Estimates place the population mean between 500-1,000 individuals (Modde et al. 1996 and Lanigan and Tyus 1989). The likelihood of a single, small, and possibly declining population provided impetus for examination of other potential spawning azeas to augment limited recruitment found in the middle Green River population (Modde et al. 1996). Adult razorback sucker have been collected in the lower Green River as part of other studies (Tyus 1987; Tyus et al. 1987; McAda et al. 1994) and Gutermuth (1994) collected two early juvenile razorback sucker (36.6 and 39.3 mm total length [TL]) near Hells Roaring Canyon (RK 89.7, RM 55.7) in 1991. Collection of these juveniles provided the first evidence of razorback sucker survival beyond the larval stage since Taba et al. (1965) and stimulated the seazch for spawning azeas Lower in the system. Razorback sucker aze 7-9 mm TL at hatching and 9-11 mm TL at swim-up. In rivers, after larvae emerge from spawning substrates they aze transported downstream to nursery azeas (e.g., flooded tributary mouths, backwaters, floodplain wetlands; Muth et al. 1998). Nursery azeas in the Labyrinth Canyon region of the lower Green River were tazgeted due to captures of 1221azva1 razorback sucker in two habitats in 1993 (Muth and Wick 1997). At the time, these collections represented the largest number of razorback sucker larvae captured in the lower Green River.