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Razorback sucker spawning also has been observed in riverine sections of the Colorado River below Hoover Dam (Minckley 1983). Mueller (1989) gave the following account of reproductive behavior in that riverine habitat: "Spawning behavior was similar to that reported for populations in reservoirs. However, spawning appeared to be less mobile in the river. The majority of fish, which appeared to be "small" (approximately 40 cm total length) males, maintained stationary positions on the downstream end of the site. This behavior was different from the roving nature previously observed and reported for reservoir-spawning groups (Minckley, 1983). Larger fish, presumably females, periodically moved into the area from the adjacent river, attracting some of the otherwise stationary males to form spawning groups of three to eight fish. -These groups, composed of one female and several males, would spawn over depressions or swim around the area before dispersing. The spawning act only took a few seconds and, other than orienting with the current was similar to that reported elsewhere. However, on several occasions spawning groups appeared to seek shelter downstream of large boulders and, while maintaining their position, would roll in mass for several seconds." Habitat Preferences Adults. Habitat selection by adult razorback suckers changes seasonally. Tyus and Karp (1990) detected movements of adult fish into flooded areas in early spring, and suggested that flooding of bottomland during spring runoff was important to adults for feeding and for temperature regulation. The flooding of bottomland also supplies allochthonous input to the river, which may subsequently provide food for one or more life stages. Radiotelemetry showed that adult fish in the Green and Duchesne rivers, Utah, selected deeper near-shore runs during the spring, but shifted to relatively shallow waters of submerged mid-channel sandbars during the summer months. The fish occupied locations with water depths ranging from 0.6 to 3.4 m (2.0 to 11.0 ft), water velocities of 0.3 to 0.4 m/s (1.3 to 2.0 ft/s), substrates of sand or silt (Tyus 1987). During summer, the fish occupied midchannel sand bars where the water was less than 2 m deep and velocity averaged 0.5 m/s (Tyus 1987). These bars consisted of small, underwater dunes and depressions in which the fish may have been feeding on trapped allochthonous material (Tyus 1987). In winter, radio-tagged razorback suckers used slow runs, "slack waters" and eddies, in depths of 0.6 to 1.4 m (2.0 to 4.6 ft) and velocities of 0.03 to 0.33 m/s (0.1 to 1.1 ft/s; Valdez and Masslich 1989). In the upper Colorado River, near Grand Junction, Osmundson and Kaeding (1989) reported similar habitat use: pools and slow eddies from November through April, runs and pools from July through October, runs and backwaters during May, and backwaters, eddies, and flooded gravel pits during June. Selection of depths changed 10