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resulted in channel degradation (Lyons 1989). As the river cuts down into the bed of the channel, wetlands and riparian areas can lose their hydrologic connection to the river. Loss of this connection can dry up the riparian areas or reduce water levels so that floodplain habitats are unavailable to the fish. Changes in the hydrograph also can lead to changes in channel geometry. Reduction in channel width has increased the average velocity in the main channel and decreased the number of low-velocity backwaters (Wick et al. 1982). Important backwater and low-velocity shoreline habitats have been eliminated through siltation and subsequent vegetative growth (Wick et al. 1982). In particular, river shorelines have been altered by establishment of the exotic plant tamarisk (Tamarix chinensis}. In Canyonlands National Park, the establishment of tamarisk on islands, sandbars, and river shorelines has decreased channel width by an average of 25% (Graf 1978). Physical structures that have altered flow regime also may be barriers to fish movement. In the Colorado and Green rivers above Glen Canyon Dam, there are five structures which completely block fish movement and two others that block fish movement either partially or seasonally (Burdick and Kaeding 1990). On the San Juan River in New Mexico, there are five diversion structures with the potential to impede fish movement (Platania et al. 1991). The lower basin has at least 15 mainstream dams that block fish movement on the Colorado, Gila, Verde, and Salt Rivers. This accounting is by no means complete, but demonstrates that water development projects have greatly fragmented fish habitat, thus interrupting life cycles. Flow regulation has had indirect, but significant, effects on water quality in the Colorado River system. The native fish fauna evolved in a warmwater system in which there was extreme seasonal variation in suspended sediment concentration. Reservoirs in the system now trap large quantities of sediment and release clear water. The reduction of sediment load may have effects on the fish fauna that go beyond the alteration of channel geometry. Increased water clarity may have increased vulnerability of younger life history stages of the razorback sucker through predation by introduced, visual predators. The large impoundments also have had a significant effect on river temperatures. The impounded lakes stratify seasonally and typically release cold hypolimnetic water. The cooler water temperatures resulting from dam operations may exclude endangered fishes from portions of their original range (Vanicek 1967). For example, adult razorback suckers prefer water temperatures between 22-25°C (71.6-77°F) and may avoid water temperatures below 14.7°C (58.5°F) and above 27.4°C (81.3°F} (Bulkley and Pimental 1983). Winter water temperatures drop well below this reported preference range throughout most habitat occupied by razorback sucker in the upper basin, but summer temperatures are generally within the preferred range. However, there are two reaches of the Green and Colorado rivers where spring and summer temperatures are clearly below the preferred range of razorback sucker. The fish is virtually absent below Flaming Gorge Reservoir for 105 km (65 mi) where summer 18