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descending limb of the hydrograph, closer to the peak in low flow years. The spawning period typically commenced in June and lasted 3-4 weeks and as long as seven weeks in 1996. ^ YOY Gila spp. daily growth (mm/day) varied from a low of 0.15 above the canyon during August 1995 to a project high of 0.83 recorded during August 1994. Daily growth was positively correlated (RZ = .59; p=0.02) with respective monthly degree days > 20°C . ^ Catch rates (fish / mz) of YOY Gala spp. were greatest in the above canyon sub-reach. However, catch rates in both the above canyon and canyon sub-reaches were significantly higher than those found in the sub-reach below canyon. Adult sampling below the canyon in 1992 revealed low densities of roundtail chubs and no adult humpbacks. ^ Spawning success as indexed by the CPE was more variable in the above canyon (roundtail f chubs) sub-reach than within Westwater Canyon (roundtail and humpback chub), indicating that the humpback chub spawning success may be less dependent on magnitude of flows - in part describes reproductive isolation of native Gila spp. ^ There were strong positive correlations found between the previous year's peak flow and July catch rates of YOY both above the canyon (R2= .98; p= 0.016) and within Westwater Canyon (R2= .97; p= 0.004). The high flows of 1993 and particularly 1995 may have prepared spawning habitats or significantly reduced nonnatives to the benefit of chubs in Westwater Canyon. YOY CPE was negatively correlated with the amount the observed mean monthly flow in June deviated from the historic June average (as measured at the Cisco gage)(e.g., August CPE above the canyon (Rz= .98; p= 0.008) and within the canyon (Rz= .47; p= 0.42)). ^ Chub reproductive success was maximized when the Colorado River peaked near 30,000 cfs in 1996. When considering the five experimental data points (five spring peaks) there was an upper and lower flow threshold in regards to reproductive success. Analysis by sub-reach indicates the optimum flow window for roundtail chub may encompass higher flows than humpback chub. [This simple relationship is confused when considering the positive correlation between CPE and previous year peak - 1995 was the highest flow year in this study period.] Based on this and the previous conclusion there appears to be a real benefit of peak flows at the Stateline gage of 30,000 cfs (once every two years} and a flow in excess of 40,000 cfs (one in 3 years). ^ Multinomial analyses for YOY chub habitat preference indicated no selection when testing the presence /absence of YOY in sampled habitat types. A similar analysis incorporating densities of YOY indicated chubs were not equally distributed among those habitats. YOY chubs selected for backwaters above Westwater Canyon and selected for embayments within the canyon. Our observations in the field indicated that YOY chubs in the canyon used whatever habitat they could find. In a canyon like Westwater, where relatively small changes in flow result in large changes in stage, specific habitat stability is poor. If larval and YOY chubs are going to persist in Westwater Canyon, and they do, they need to be opportunistic in their habitat use. This non- specific use of low velocity habitat was likely more common in G. cypha than G. robusta. Based on our habitat analysis, it appears YOY roundtail chub (and nonnative cyprinids) do better during low flow years in Westwater Canyon - in part describes reproductive isolation of native Gala spp. and explains trends in longterm monitoring data. Day and Crosby (1997) also found that YOY chubs did not select habitat types preferentially. 28