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Razorback suckers are uncommon in turbulent canyon reaches (Lanigan and Tyus 1989; Minckley et al. 1991). Although these canyons may not be suitable for spawning, nursery, or rearing areas, they provide corridors for movement and migration between more typically occupied habitat. Additionally, larger tributaries within canyons may provide suitable habitat to complete life history requirements. These canyon reaches also provide corridors for larval drift, (e.g., Whirlpool Canyon below an identified spawning site in the lower Yampa River; Tyus and Karp 1990). The ability for razorback suckers to move between spawning, adult, and nursery habitats is essential for completion of all life history requirements. COLORADO SQUAWFISH (Ptychocheilus lucius, Girard, 1856) Colorado squawfish are adapted to rivers with seasonally variable flow, high silt loads, and turbulence. The Colorado squawfish is the largest member of the minnow family in North America, with maximum weights historically exceeding 80 lbs. The Colorado squawfish is endemic to the Basin. It is the largest of four existing species of the genus Ptychocheilus (Moyle 1976). Its origins predate recorded history, but fossils indicate that by the mid- Pliocene epoch (about 6 million years ago) early Ptychocheilus had riverine adaptations that were similar to modern forms. During the Pleistocene epoch (about 1 million years ago), an earlier wet climate was interrupted by periods of desert conditions (M. Smith 1981). It has been hypothesized that the migrations reported for Colorado squawfish are a perfect life history strategy for the survival of a large predaceous fish in the historic Colorado River environment (G. Smith 1981; Tyus 1986, 1990). During the spawning season, adult Colorado squawfish have been known to migrate up to 200 miles upstream or downstream to reach spawning areas (Tyus 1990). HISTORIC DISTRIBUTION AND ABUNDANCE Historically, Colorado squawfish occurred throughout the Basin (Figure 3). In the Lower Basin, Colorado squawfish were recorded in the Colorado River mainstem from the Gulf of California in Baja California del Norte to Lee's Ferry in Arizona, the Little Colorado River, and the Gila River and its tributaries, the San Pedro, Salt, and Verde rivers. Colorado squawfish also were recorded in the Salton Sea, which was sporadically filled with water from the Colorado River (Follett 1961; Minckley 1973, 1979, 1985; Moyle 1976, Gobalet 1992). In the Upper Basin, Colorado squawfish have been captured in the lower reaches of the Gunnison, White, Yampa, Little Snake, Dolores, San Juan, Duchesne, Uncompahgre, and Animas rivers (Jordan 1891; Ellis 1914; Beckman 1952; Lemons 1954; Koster 1957, 1960; Johnson 1976; Valdez et al. 1982a; Platania 1990; Marsh et al. 1991), and Plateau Creek, a tributary of the Colorado River (Bob Burdick, USFWS, pers. comm.). The species also was common in the Colorado and Green Rivers mainstems, plus probably numerous smaller streams (Ellis 1914; Bosley 1960; LaRivers 1962; Baxter and Simon 1970; Holden 1973; Seethaler 1978; Johnson and Oberholtzer 1987). 16