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" disconnected from the river channel either by man-made revetments or by sand bars or encroaching riparian vegetation that are no longer scoured owing to truncation of peak: flows by regulation (e.g., Graf 1978, Stanford and Ward 1986a). Indeed, I believe loss of productive backwater environments may in part explain why humpback chub are found only in canyon segments and why , . razorback sucker and squawfish move around a great deal. Food webs associated with gravel bars are very likely more productive and pennanent (e.g., Ward and Stanford 1991), and the larger razorback sucker and squawfish adults must search for these more productive sites, owing to their large size and need for abundant, large forage items. Squawfish adults may be most commonly found in or near the rhithron-potamon transition zone (Figure 5) because the trcUlsition zone is the only area with sufficient productivity and a pennanent food web to support the life history energy balance of this large predatory animal. Indeed, other native fishes that are the natural prey of adult squawfish, especially roundtail chub and bluehead sucker (Catostomus discobolus), are more abundant in or near the transition zones (Doug Osmundson, U.S. Fish and Wildlife Service, Grand Junction, CO, personal communication) where algae and zoo benthos forage likely is most abundant The trophically dynamic nature of the potamon reaches of the Upper Colorado River Basin and the interactive influences with geomorphic controls are poorly understood aspects of the ecology of the endangered fishes. On the one hand, it is apparent that these fishes prefer low velocity habitats; on the other, these low velocity habitats may not be as productive as higher velocity reaches owing to fluctuating flows caused by regulation. Measurements are needed to more firmly establish cause and effect. The problem is complicated by the fact that site-specific velocities vary with flow, which is precisely why channel geomorphology is so complex and dynamic in time and space. I conclude that throughout their life cycle these fishes are highly adapted to variations in flow velocity, depth, turbidity and food web structure and function associated with this spatially and temporally dynamic biophysical interaction. They simply move around as flow varies, constantly seeking the best energy return on energy invested in foraging. In the case of squawfish, their large size apparently allows considerable movement to efficiently use a 21