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areas of the Green River (Andrews 1986), because channel configurations may change dramatically in response to variable peak flows. In other words, as the channel morphology changes from year to year, a given discharge will vary in its inundation of backwaters and bottomlands which can profoundly influence fishes and other biota that must move into backwaters, flooded bottomlands and other low velocity habitats from the channel and back again in short (die!) and long (seasonal) time frames. Efforts to build process-response models of flow and physical habitat relationships (e.g., Harvey et ale in press) therefore must take into account that flow and substratum relations in most riverine environments are stochastic and cannot accurately be described by linear or logistic functions. Indeed, complex channels which promote occurrence of low velocity habitats are virtually always characterized by non-uniform flows in time and space, whereas many models (discussed in more detail below) often assume unifonn flow. Given that a relationship exists between flow dynamics and availability of various physical habitats preferred by the fish, what role do these habitats play in the trophic ecology of the river? Except during periods.of high turbidity, the rivers in the Upper Colorado River Basin in general are intensely autotrophic and capable of supporting very productive benthic food webs on cobble substratum of riffles in the steeper segments (Annear 1980, Annear and Neuhold 1983, Carter and Lamarra 1983, Ward and Stanford 1991). Although it is not conclusively 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: a) the flux of clear, nutrient rich water through them from hyporheic sources (Figure 2) and b) wanner temperatures than occur in the channel, both of which are associated with the approach of baseflows in summer. However, channel areas in alluvial segments are not likely as productive, owing to the unstable 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, recruibnent 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 SUIVey, Boulder, CO, personal communication), which may limit 19