Colorado squawfish (Ptychocheilus lucius) and razorback sucker (Xyrauchen texanus) were formerly widespread and abundant in large streams and rivers throughout the Colorado River Basin (Miller, 1961; Minckley, 1973; Holden and Wick, 1982). Self-sustaining populations of Colorado squawfish are extant in the Upper Colorado River Basin (Holden and Wick, 1982) and are federally listed as endangered (United States Department of Interior, 1974). Razorback sucker, proposed for federal listing as an endangered species (United States Department of Interior, 1990), occurs sporadically in the Upper Colorado River Basin (Tyus, 1987; Lanigan and Tyus, 1989) and in some mainstream Colorado River reservoirs in the Lower Colorado River Basin (Minckley, [...]