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INSTREAM FLOWS TO ASSIST THE RECOVERY OF ENDANGERED FISHES 13 Upper Colorado River Basin, in general, are in- tensely autotrophic and capable of supporting very productive benthic food webs on cobble substratum of rifles in the steeper segments (Annear 1980; Annear and Neuhold 1983; Carter and Lamarra 1983; Ward and Stanford 1991). Although not con- clusively documented in the Upper Colorado River Basin, backwater environments, which are most abundant in the alluvial segments, are apparently very productive after spring runoff owing to the flux of clear, nutrient-rich water through them from hyporheic sources (Fig. 2) and warmer tempera- tures than occur in the channel, both of which are associated with the approach of baseflows in sum- mer. However, channel areas in alluvial segments are probably not as productive owing to the unsta- ble nature of the sand and mud bottoms (Ward et al. 1986; Ward and Stanford 1991). Moreover, as one moves downstream toward Lake Powell on either the Colorado River or the Green River, recruitment of fine sediments increases. The lower reaches of both rivers are characterized by extensive deposits of silt and clay (E. D. Andrews, U.S. Geological Survey, Boulder, Colorado, personal communica- tion), which may limit zoobenthos production. In- deed, zoobenthos species richness and biomass de- cline downstream from the rhithron-potamon transition zone as the river bottom changes from coarse to fine substratum (Carter and Lamarra 1983; Ward and Stanford 1991). These studies and discussions with researchers indicate that food webs are more stable, complex, and productive in the upstream reaches of the potamon, associated with cobble substratum within the channel (e.g., Yampa Canyon, 15-mile reach, lower Gunnison River). In the alluvial seg- ments of downstream reaches on the Green and Colorado rivers, productive food webs may only be present in low velocity backwaters and the few cobble bars. Studies have been inconclusive as to exactly how productive backwater environments actually may be, but algae, zooplankton, and mud- dwelling midge (Chironomidae) larvae are pre- sent in backwaters on the Green River (Grabowski and Hiebert 1989). I would expect naturally func- tioning backwaters (i.e., seasonally flooded and continuously connected to the channel) to contain rooted aquatic vegetation (i.e., as opposed to en- croaching riparian vegetation), which provides substratum for algae, odonates, snails, mayflies, and caddisflies, in addition to forms living on the bottom (e.g., oligochaetes and midges). Organic detritus originating in the river channel (e.g., periphyton, drifting leaves) also may be deposited in low velocity habitats, providing substratum for detritivorous insects and fishes. Hence, backwa- ter food webs typically have abundant forage for small fish, such as YOY squawfish, which are then available to larger predators. A large body of lit- erature supports the concept that naturally func- tioning floodplain wetlands of rivers are very pro- ductive and an essential component of the life history of fishes that migrate between channel and floodplain wetlands (e.g., Welcomme 1979; Junk et al. 1989; Ward 1989; Petrere 1991). Because they fringe the channel rather than extend across it, backwaters and associated flood- plain wetlands are more ephemeral than cobble bars, which remain partially inundated even at the lowest flows. Moreover, backwater and wetland (flooded bottomland) environments in many uncon- strained (floodplain) areas of the Upper Colorado River Basin have been ecologically disconnected from the river channel either by man-made revet- ments 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 explain, in part, 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 probably more pro- ductive and permanent (e.g., Ward and Stanford 1991), and the larger razorback sucker and squaw- fish adults must search for these more productive sites because of their large size and need for abun- dant, large forage items. Squawfish adults may be most commonly found in or near the rhithron-pota- mon transition zone (Fig. 5) because the transition zone is the only area with sufficient productivity and a permanent food web to support the life his- tory 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, personal communication), where algae and zoobenthos forage probably are 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, these fishes prefer low velocity habitats; on the other hand,