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reservoir habitat of Lake Mojave (lower basin), uninfluenced by seasonal flow patterns, most spawning occurs from January to April (Minckley 1983, Langhorst and Marsh 1986, Mueller 1989). In the Green River (upper basin), spawning occurs during mid- to late May in high water years and April to mid-May in low water years (Muth et al. 1998). In the upper Colorado River, capture dates for ripe adults during a 15-year period corresponded to the period when peak snow-melt runoff flows typically occur: of 42 ripe fish, 40 (95%) were captured between May 20 and June 17, and 84% of peak flows in the Grand Valley over an 83-yr period occurred between May 20 and June 23 (Osmundson and Kaeding 1991). Migrations to spawning areas have been documented in upper basin rivers. In the Grand Valley, 2 adults migrated 11 and 26 km just prior to the estimated spawning period (Osmundson and Kaeding 1989); in the Green River, adults have migrated as far as 190 km (Tyus and Karp 1990). Razorback suckers in spawning condition are generally captured in the Green River system in one of two known mid-channel sites in riffles or shallow runs with a gravel or cobble substrate (Tyus and Karp 1990). In the lower basin, razorback suckers successfully spawn along gravel beaches of large reservoirs (Douglas 1952, Minckley 1983, Bozek et al. 1984, Mueller 1989, Holden et al. 1999). No mid-channel spawning sites have been located in the upper Colorado River. During 1974-1991, 38 of 42 adults in spawning condition captured in the Grand Valley were from flooded gravel pits (summarized by Osmundson and Kaeding 1991). It is unknown whether these fish, when caught, were staging in preparation for mid-channel spawning or whether they spawned in these off-channel habitats. McAda and Wydoski (1980) captured two ripe females and five ripe males in one trammel net in a large gravel-pit pond near Grand Junction and believed these fish were spawning at the time of capture. Availability of appropriate temperatures plays an important role in reproductive success of fishes. Incubation time and hatching success of fertilized razorback sucker eggs varies with water temperature. After egg deposition and fertilization, embryos incubate in the substrate for varying lengths of time, with the shortest times occurring at the warmest temperatures (Haines 1995). In the Green River, mid-channel spawning results in larval 9