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as for the smaller Colorado squawfish, zooplankton would likely be an important component of their diets (Joseph et al. 1977) and corresponding overlap values would be high, especially for specimens < 11 mm TL. Although dense populations may develop in backwaters, zooplankton may be limited under certain conditions because plankton communities in rivers are subject to dramatic spatial or temporal fluctuations in abundance and diversity (Hynes 1970, Welcomme 1985, Ward 1989). In support of this generalization, Grabowski and Hiebert (1989) reported that zooplankton densities were higher in backwaters than in mainchannel habitats within the upper reach and documented both spatial and temporal fluctuations in zooplankton abundance. They also observed higher concentrations of zooplankton in the more isolated backwaters than those with a broad connection to the river and suggested that densities were influenced by extent of water exchange between backwaters and the main river channel. Stanford (1993) stated that more information about ecology of native and nonnative fishes and trophic structure in nursery habitats is required to clarify important relationships between flow and backwater food webs. Seasonal flow recommendations implemented in 1992 for operation of Flaming Gorge Dam were intended to provide for low, stable baseline flows during summer and fall to protect and enhance backwater nursery habitats (US Fish and Wildlife Service 1992). However, despite reductions in the magnitude and frequency of releases from the dam, considerable diel fluctuations in flow at baseline still occur and may continue to impact the natural functioning of backwaters (Stanford 1993). Studies are needed to determine the productivity of backwaters under present regulated flow regimes, to better assess the type and strength of interactions between native and nonnative fishes in backwater food webs, and to define factors affecting these interactions. 15