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to fishes during the period of spring through early autumn. When backwaters cool to near or below mainchannel water temperature, abundance of fish decreases. Introduced fishes were generally more abundant than natives in backwaters. Of native fishes found in backwaters, flannelmouth sucker and speckled dace showed a decline in CPUE and relative abundance during the October 1985 fluctuating flows, whereas humpback chub and bluehead sucker generally increased. Results could have been much different if fluctuating flows had occurred during summer when many of these species are in their larval stages. Larval fish have much less mobility and are more vulnerable to river currents than older, larger individuals. TRIBUTARIES ` Tributaries are important in maintaining both endemic and introduced fish species of the Colorado River. Trout were captured most frequently in winter and early spring when water temperatures were lowest. Most native fish were found in tributaries in late spring and summer when temperatures far exceeded those of the mainchannel. Native fishes and the introduced trout species also demonstrated different use patterns by tributary type. Native fishes were most common in streams classified as major tributaries with low gradients and originating outside Grand Canyon, e.g. Paria and Little Colorado rivers and Kanab Creek. Conversely, trout preferred tributaries that are steep in gradient, relatively straight, and originate as springs in Grand Canyon, e.g. Tapeats, Bright Angel, and Shinumo creeks. The Little Colorado River appears to be crucial to the long- term survival of humpback chub. No evidence was found to j. indicate that major spawning areas occur outside this tributary, although as has been noted in other studies, Shinumo and Havasu creeks could support limited reproduction. Mainchannel reproduction by humpback chub is at best extremely limited, or more likely nonexistent as a result of cold water temperatures. The mixing zone formed at the mouth of the Little Colorado River appears to provide a thermal intergrade between the cold mainchannel and the warm tributary. Our collections indicate this may be an important staging area for larval chubs before entering the mainstream. -5-