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seasonally; use of relatively shallow water occurred during spring and use of deeper water during winter. Mean depths were 0.9 to 0.99 m (3.0-3.3 ft) during May and June, 1.62 to 1.65 m (5.3-5.4 ft) from August through September, and 1.83 to 2.16 m (6.6-7.1 ft) from November through April (Osmundson and Kaeding 1989). Adult razorback suckers use a great variety of habitats, including lower gradient, low- velocity riverine sections of canyon-bound areas. The fish also have been tracked moving through whitewater habitats (Tyus and Karp 1990), but spent little time there. There are few historic records of razorback suckers in Grand and Marble canyons of the lower Colorado River, possibly due to lack of historic sampling in these inaccessible whitewater canyons (Minckley et al. 1991). Lanigan and Tyus (1989) suggested that razorback sucker distribution in the Green River may be constrained by whitewater canyons that either impede migration or do not have suitable habitat. Although the fish has been extirpated from its historic riverine habitats in the lower Colorado River basin, the species never may have been common in whitewater canyons there (Bestgen 1990). As an example, historic locations occupied by the fish in the Gila and Verde rivers lacked extensive whitewater areas. Razorback suckers also utilize reservoir habitat, where the adults may survive for many years and habitat use of adults and larvae have been thoroughly studied (Minckley et al. 1991). The fish move throughout Lake Mojave and other reservoirs, where they use a variety of habitats (e.g., Medel-Ulmer 1983, Minckley 1983, Marsh and Minckley 1989, Minckley et al. 1991). Habitat preferences of adult razorback suckers reared in hatcheries and implanted with ultrasonic transmitters also have been studied in the complex environment of the lower Imperial Division of the Colorado River in Arizona and California. The fish used all habitat types, but preferred backwaters and the main impoundment (Bradford et al. 1998). Larvae and juveniles. Habitat use of small life stages of the razorback sucker have not been studied in riverine environments. Marsh and Langhorst (1988) observed that larval razorback suckers in Lake Mojave remained near shore after hatching, but disappeared within a few weeks. Young hatchery-produced fish remain along shorelines, in embayments along sandbars, or in tributary mouths, and later disperse into the main channel or larger backwaters (Minckley et al. 1991). One laboratory- study indicated that 2-week old larval razorback suckers actively, rather than passively, entered the drift and moved primarily at night (Paulin et al. 1989). The tendency to enter the drift suggests that the species takes advantage of downstream transport for moving from spawning to nursery habitats, which are presumed to be ephemeral shoreline habitats. Tyus (1987) reported captures of young razorback sucker larvae from a backwater immediately downstream of a known spawning area, and during a four year period (1992-1996), 1,735 drifting larvae were captured from the mid and lower Green River using a variety of sampling gear (Muth et al. 1998). Habitat needs of juvenile razorback suckers are not well known because juveniles are not commonly encountered, especially in riverine habitats (Tyus 1987). Most 11