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002492 or mature HBC. In contrast, high mainstream flows may affect younger HBC (YOY, larvae). High flow impacts are likely to most pronounced from May through July, as larval and young HBC emerge from the LCR and occupy mainstream near-shore and backwater habitats. Although a 2-4 day BHBF may briefly create an additional pool area at the mouth of the LCR, that effect is unlikely to substantially benefit drifting HBC, that would subsequently be flushed into the mainstream. Flows of 45,000 overtop existing bars in the LCR area, and subject shoreline and backwater habitats to cold temperatures and high velocity flows. Thus, high flows stress and displace young HBC in those habitats. Therefore, even brief BHBFs from May through July may negatively affect HBC recruitment throughout the mainstream. Rainbow trout Adult rainbow trout distribution varies above and below the LCR, but there appears to be little downriver migration of these fish from the trout fishery at Lees Ferry. The foodbase for trout varies with age. Larvae-fingerlins are reported to consume immature chironomids, cladocerans, and zooplankton. Adult trout consume Cladophora, chironomids and Gammarus associated with Cladophora. Habitat utilized by adults in descending order include runs, eddies, riffles, pools and backwaters. Fish select for refuge areas during high flows, moving from fast current environments to slow current velocity environments (Agrandi et al 1992; McKinney et al1996b). Spawiiing occurs from November through spring (April) in the Lees Ferry Reach. Steady flows that allow access to spawning bars positively affects spawning, conversely fluctuating releases that dewater shallow gravel bars potentially reduce spawning success at these locations. Trout show an affinity for specific spawning sites (Agrandi et al1992, so that impacts to spawning sites should be a consideration concerning high flow and fluctuating releases. Stranding occurs most often during spawning periods and when releases fluctuate rapidly. Overtime, high flows may scour or redistribute spawning gravels utilized by this species. Fry come off redds from January through May. The number of fry reach maxima in electrofishing samples during the spring and fall, reflecting extended spawning periods. Fingerlings are present throughout most of the year. High flows did not show a significant loss of fingerlings in the Lees Ferry population (McKinney et al1996b). Small fish (fry and fingerlings) show affinity for low velocity near-shore habitats. High scouring flows may transport small fry downstream, but this has not been documented. Fingerlings and adult seek cover from high velocity flows. Little or no downstream displacement of fish was apparent due to the experimental spate of 1996. Avifauna Waterbirds are largely found in the Lees Ferry and Marble Canyon Reaches, likely due to the aquatic food base associated with the dam. Mallards and mergansers appear to be resident constituents, while many other waterfowl use the reach for overwintering, or are transient species (Stevens et al 1997). Seasonality and distance downstream influences abundance of waterbirds: winter is reported to be a time of year when waterfowl numbers are greatest, and downstream aquatic foodbase availability decreases (Stevens et al 1997).