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collected from middle Green River spawning sites, indicating they likely work spawning substrates (Figure 5b). Direct observations of riverine spawning on cobble substrates on the lower Colorado River below Hoover Darn were reported by Minckley (1983) and Mueller (1989). Razorback sucker also spawn on cobble substrates in lake environments (Minckley, 1983). Razorback females worked cobble substrate at Lake Mohave with their anal and caudal fins. Sometimes noticeable depressions are formed in the shoreline cobble deposits. System instability and alteration of fish habitat The historical river channel was broader with a more active channel and floodplain than today (Graf 1978). Prior to extensive dam building that occurred on the upper Colorado River system during the 1960s, human impact and varied climatic and flow cycles worked in concert to cause river channels to respond in complex ways (Gellis et al. 1991). During the late 1800s and early 1900s a period of increased flows and sediment production occurred (Figure 6 and 7). This was followed by a period of moderate flows, reduced sediment discharge and the appearance of more persistent, introduced riparian vegetation (salt cedar) in the lower Colorado River system. Gellis et al. (1991) noted that suspended-sediment decreased after the early 1940s in the Colorado Plateau portion of the Colorado River Basin although discharge of the major rivers did not change significantly (Figure 7). They attributed the reduced sediment loads to both decreased sediment production and sediment storage in channels of tributary basins (Gellis et al. 1991). Salt cedar progressively colonized banks in an upstream direction at a rate of about 20 km yr ` (Graf, 1978). The salt cedar spread at a consistent rate from the Colorado River near Lee's Ferry, Arizona in 1910 to Browns Park on the Green. River upstream from Dinosaur National Monument by 1950 (Graf 1978). 15