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18 I I I I I I I I I I I I I I I '-1 I I I Colorado River clade. The Snake River clade revealed a strongly supported Lahontan group that did not share haplotypes with surrounding basins. It contained instead scattered sites from former Pluvial Lake Lahontan, as well as from eastern California. It was, in turn, sister to the Owens River, while R. falcatus was sister to this larger clade. The hypothesis of a southerly, "fishhook" -configured tributary associated with a westward-draining Pliocene Snake River was manifested by the relationship of this Lahontan clade to upper Snake and northern Bonneville localities. The Klamath/Pit and Columbia rivers were sisters and their basal relationship to all the above, and this supported the hypothesis of a pre-Pliocene western passage of the Snake River. Our data also supported at least three separate ichthyofaunal invasions of California, as well as a Bonneville Basin fragmented by a north-south connection between southeastern Idaho and the Colorado River. The dual western and southern movements of R. osculus from southern Idaho suggested a northern origin for the species, possibly associated with Tertiary Lake Idaho. Habitat The Use of Topographic Models to Predict the Effect of Stage Changes on Physical Backwater Characteristics John Hayse', Kirk LaGory', Brian Cantwell I , Steve Railsback:!, and August Deschais:! I Argonne National Laboratory, Argonne, Illinois.. 1 Lang, Railsback, and Associates, Arcata, California To protect backwater habitats used by Colorado pikeminnow (Ptychocheilus lucius) juveniles, flow recommendations for the Green River call for limiting within-day stage changes to 0.1 m or less at the Jensen gage during the baseflow period. We developed topographic models for six representative backwaters in the Ouray reach of the Green River to evaluate the effects of stage changes on their physical characteristics. Standard surveying techniques were used to obtain detailed topographic information for backwaters from 19 to 21 August 2003. Stage data in the study area were collected using a temporary stage recorder. Using topographic data, contour maps and GIS models for each backwater area were developed to predict the area, volume, and depths for each backwater as a function of main channel water stage. Maximum depth of backwaters ranged from 0.5 m to 2.4 m (mean maximum depth = 1.2 m, SD = 0.7), surface area ranged from 220 m2 to 1744 m2 (mean surface area = 779 m2, SO = 582.7), and volume ranged from 59.5 m3 to 853.7 m3 (mean volume = 349.4 m3, SD = 399.4) at the time that topographic data were collected. For the study backwaters, a 0.1 m decrease in stage would result, on average, in an II % decrease in maximum depth, a 19% decrease in surface area, and a 30% decrease in backwater volume. This information is useful for evaluating relationships between main channel flow conditions and physical conditions in