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Filial Report 3-43 September 2000 Table 3.13.-In-channel response of sandbars to various flood levels in the Green River.3 Effect of Flood Magnitude on In-Channel Habitats Elevation of Existing Bar Tops Less Than Bankfull Greater Than Bankfull Less Than Bar-Top Flood Low Rearranges habitats. Net change in habitat availability unknown. Maintains the increased availability of deep habitats. Maintains or decreases availability of shallow habitats. High Increases shallow habitat availability. Decreases deep habitat availability. (1994 runoff peak) Rearranges habitats. Net change in habitat availability unknown. Increases availability of deep habitats. Decreases availability of shallow habitats. (1993 runoff peak) Maintains the increased availability of deep habitats. Maintains or decreases availability of shallow habitats. a Effects in boldface were measured; those in italics were modeled. Unmodeled and unmeasured predictions are shown in normal typeface. Source: Rakowski and Schmidt (1999). The larger flood peak of 1993 increased the height and range of the elevations of the nursery habitat's bed, thus increasing the flow at which habitat availability was maximized and broadening the range of flows at which habitat was available (Rakowski and Schmidt 1999). The lower flood peak of 1994 decreased the range of bed elevations by scouring the higher elevations and filling the lower elevations, thus narrowing the range of flows at which habitat was available and shifting the peak of habitat availability to a lower elevation and flow. A series of low-peak floods would continue this process until the discharge that maximized habitat availability was quite low. In 1992, after six years of drought, habitat availability was maximized at 35 m3/s (Rakowski and Schmidt 1999). Eddies are another important component of low-velocity habitat in the Green River, but these habitats form behind geomorphic features (e.g., debris fans, large rocks) that are more resistant than sediment bars to annual peak flows. In Desolation and Gray Canyons, increases in flow change the distribution and type of eddy habitat present, but the total area of eddy habitat changes little (Orchard and Schmidt 2000). At any given flow, approximately 25% of the shorelines occur within eddies. At base flow, small frequent shoreline eddies make up most of the eddy habitat and increase in frequency between 59 and 198 m3/s. As flow increases from 198 to 765 m3/s, large, infrequent eddies formed by constrictions in the channel make up the majority of eddy habitat. Although the availability of low-velocity shoreline habitat apparently changes little in Desolation and Gray Canyons with changes in flow, habitat conditions as determined by substrate characteristics in those habitats may change considerably (Orchard and Schmidt 2000). Low flows produce highly complex shoreline habitats with mostly bare sand and gravel substrates. Higher flows