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I, ~: : I': Ii: I " " , o C.> ",:! i....... 1\.) -..l significant. For red shiner, seasonal abundance differences were not significanl, but differences among years were. Seasonal, reach, and annual comparisons did not yield significant differences for fathead minnow. Seasonal and annual (reaches combined) differences in speckled dace abundance were significant as was that among years (seasons combined). Flannelmouth sucker abundance relationships were similar to those for speckled dace, Only annual comparisons (reaches combined) of blue head sucker abundance yielded significant differences, Abundance of western mosquitofish was significantly different among years (reaches or seasons combined). Abundance of red shiner, fathead minnow, flannelmouth sucker, and speckled dace generally declined (both summer and autumn) from 1993 through 1997. Abundance of blue head sucker and western mosquitofish were variable with no evident pattern. Total species abundance was not a good predictor of total autumn abundance, However, summer abundance of fathead minnow, speckled dace, flannel mouth sucker, bluehead sucker, and western mosquito fish was a good predictor ofthei~ autumn abundance, Autumn abundance of red shiner and channel catfish was not related to their summer abundance. An array of biotic and abiotic factors influenced the seasonal abundance of fishes in secondary channels of the San Juan River. Elevated flows during spring runoff made most secondary channels accessible to all fish residents oflhe river, particularly large- bodied individuals. Large-bodied fishes may enter secondary channels to forage, spawn, and avoid higher velocity water of the primary channel. Habitat preferences and life history strategies likely influenced the longitudinal abundance distribution oflarge- bodied fishes in secondary channels. The rarity of Colorado pikeminnow and razorback sucker in secondary channels during spring was likely more a consequence oftheir overall rarity rather than avoidance of secondary channels. During summer and autumn, fish assemblages of secondary channels are numerically dominated by small-bodied fishes and young oflarge-bodied fishes. Among these species, nonnative fishes were typically the most abundant. There was generally a posilive relationship between summer abundance of several common species, including red shiner, and elevaled spring discharge. Although a natural flow regime was mimicked, spring flows during lhe study were never as high as occurred in some years prior to completion of Navajo Dam. Thus, elevated spring flows evidently were not sufficient to displace some nonnative species, as documented in several other southwestern systems having a natural hydrograph. Elevated flows during summer, however, generally had negative effects on autumn abundance of nonnative fishes and positive or no effect on native fishes. Abundance of all commonly collected species was less in 1997 than in 1993, The low spring runoff and low summer and autumn flows of 1996 depressed the abundance of all species, nalive and nonnative. Abundance of most increased slightly in 1997 with higher spring runoff and higher summer and autumn flows, but not to levels found prior to 1996. Although the abundance of mosl species changed with mimicry of a natural hydrograph, il is likely changes will continue as flows continue 10 mediate species relationships and interactions and habitats change in response to a mimicked natural hydrograph. iv