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area did increase to that point. Flows in excess of 9000 cfs rarely occurred during the July and August nursery period, but should be avoided, if possible, to maximize native roundtail chub recruitment. Westwater Canvon A greater diversity of low velocity habitat types was found and sampled in Westwater Canyon than upstream. At the presence/absence level of detection, a multinomial analysis indicated YOY chubs utilized habitats as they were available, i.e., no selection. A closer look at the densities of YOY chubs by habitat type (Figure 9) reveals some habitats were preferred. Interestingly, the density of YOY chubs in backwaters was statistically less than expected. Embayments, characterized as pocket water areas or concavities in the shoreline geology, held greater densities of YOY chubs than was predicted. A greater percentage of effort was expended sampling shoreline habitats in the canyon than upstream - a habitat that was utilized by YOY chubs as it was available. Backwaters were negatively (not significantly) correlated with sample flows. Embayments were f available at a wide range of flows and were positively correlated with flows at the time of sampling. Although very few larval chubs were collected throughout this study, it is likely these embayments or shoreline pocket waters that provided habitat for the very early life stages of native Gila spp. during the high flows were associated with the descending limb of the hydrograph. Embayment habitat availability was a function of instantaneous flow /river stage. These habitats are not formed by sediment transport, but rather a simple process of the river inundating or stranding shoreline geology. One of the objectives of this study called for a description of the relationship between geomorphological processes of sediment transport and nursery habitat formation and maintenance in Westwater Canyon. We did not coordinate with a geomorphologist in this study. UDWR did contract with geomorphologists to describe the relationship of low velocity habitat (eddys) and flow in Desolation / Gray Canyon where numerous backwaters form in eddy return channels as flows drop in the summer (Orchard and Schmidt 1998). The short stretch of Colorado River flowing through Westwater Canyon characterized by steep gradient and confined channel configuration, is not an area of sediment deposition. The availability of backwater habitat was negatively correlated with the magnitude of the spring peak, but was more likely a function of late summer flows. It was our observation that the magnitude of the spring flood had very little effect on the availability of low velocity habitat in general in Westwater Canyon. Backwaters and other habitats associated with sand bars were sufficiently scarce that it quickly became apparent that humpback chub could not depend on them greatly as nursery areas. A multinomial analysis of habitat selection by young chubs indicated they were not used as much as was statistically expected. [If future studies indicate humpback chub spawn on cobble bars in Westwater Canyon, a geomorphological perspective of how those habitats are maintained could be warranted.] Our observations were that YOY chubs in the canyon used whatever low velocity habitat was available. In a narrow 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. Day and Crosby (1997) also found that YOY chubs did not select habitat types preferentially in Deso/Gray Canyon. Arizona Game and Fish Department (1996) YOY surveys in the Grand Canyon found higher percentages (not higher densities) of humpback chub in the beachfront species assemblages (similar to our shoreline designation) than in backwaters. This opportunistic use of low velocity areas is likely a life history strategy more common of G. cypha than G. robusta. High densities of YOY roundtail chubs were found in backwater habitats upstream of Westwater Canyon. The relationship between the availability of low velocity habitats and summer flows in Westwater Canyon reported herein, indicates that roundtail chub (and non-native cyprinids) likely do better during the low flow years. This finding, in part, describes the mechanism of reproductive isolation for these two native chubs, which coincides with the trends in the long term monitoring data sets. 23