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zoo benthos production. Indeed, zoobenthos species richness and biomass declines downstream from the rhithron-potamon transition as the river bottom changes from coarse to [me substratum (Caner and Lamarra 1983, Ward and Stanford 1991). These studies and discussions with researchers suggest 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 segments of downstream reaches on both the Green and Colorado Rivers, productive food webs may only be present in low velocity backwaters and the few cobble bars. Studies to date are inconclusive as to exactly how productive backwater environments actually may be, but algae, zooplankton and mud-dwelling midge (chironomidae) larvae are present in backwaters on the Green River (Grabowski and Hiebert 1989). I would expect that naturally functioning backwaters (Le., seasonally flooded and continuously connected to the channel) to contain rooted aquatic vegetation (i.e., as opposed to encroaching riparian vegetation, discussed below) which provide 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, backwater food webs typically have abundant forage for small fish, such as YOY squawfish, that are in turn available to larger predators. A large body of literature supports the concept that naturally functioning floodplain wetlands of rivers are very productive and an essential component of the life history of fishes that migrate between channel and floodplain wetlands (e.g., Junk et al. 1989, Ward 1989). Because they fringe the channel rather than extend across it, backwaters and associated floodplain 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 unconstrained (floodplain) areas of the Upper Colorado River Basin have been ecologically 20