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product of the relatively high abundance of progeny produced in those reaches rather than ingress of larvae from upstream. Cool-water tolerant mountain whitefish was most abundant in upstream reaches, likely because abundance of adults was highest in cool reaches upstream of there. Fishermen accounts support the idea of large upstream populations of adult mountain whitefish (pers. comm. J. Bundy). Highest relative abundance of mountain whitefish in spring samples was due to hatching of autumn-spawned eggs. Lower abundance in summer and autumn was likely due to mortality and, perhaps, intolerance of warm water in downstream reaches. Native fish relative abundance and density in summer was highest in 2002 in the Green River by a considerable margin. The pattern was supported in both the mostly regulated reach upstream of the Yampa River and in the regulation-attenuated reach downstream of the Yampa River. Spring runoff and summer base flow levels were similar among all years and summer thermal regimes were warm in each year. Thus, flow or thermal regimes in each year did not explain differing summer abundance of native fishes in the Green River. Higher summer abundance of native bluehead sucker, flannelmouth sucker, and speckled dace in 2002 did not result in higher autumn density, compared to other years. Autumn density of native suckers was similar to the low densities of flannelmouth sucker and bluehead sucker observed by Haines and Tyus (1990) in Green River backwaters. Additional years of sampling in more varied environmental conditions may be needed to determine factors that influence seasonal and annual differences in native fish abundance (e.g., Propst and Gido 2004). Presence and severity of turbidity events in the Green River may be an explanation for the relatively high variability in summer season abundance of native fishes observed in seine samples collected from 2002 to 2004. We observed relatively high native fish abundance in seine samples in summer 2002 when no severe turbidity events were observed. Alternatively, in 2003 and 2004 we observed low native fish abundance in summer and two severe turbidity events occurred in each year. Turbidity events may cause fish to lose orientation in turbid water and be transported downstream. This hypothesis was supported by high catch rates of fish in drift net samples in the Green River in 2003 and 2004 after flood events (Fig. 43). This also 44