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Early Runoff Spike Flows An alternative hypothesis that may explain some decreases in overwinter survival index in the Green River is the occurrence of early runoff spike flows that usually occur in March, prior to the normal spring runoff of late May and early June. These spike flows are usually caused by early elevation snowmelt runoff and ice breakup on the river. In all years sampled (1987-1995), a dramatic spike flow occurred just prior to the spring sampling period. Shortly after these spike flow c\ents. Valdez and Cowdell (1994, 1995) noted higher densities of age-O Colorado pikeminnow in the Lake Powell inflow than anywhere else in the upper basin. These findings suggest that young Colorado pikeminnow may disperse during these early runoff spike flows as part of their natural survival strategy for moving to warmer, more productive downstream regions of river. The formation of Lake Powell has blocked that migration corridor and reduced the survival of these young fish. If this hypothesis is proven, at least a portion of the annual loss of age-O Colorado pikeminnow from the Green River is attributed to natural phenomena and unrelated to dam operations. Nevertheless, this dispersal strategy may also be manifest during winter, in which case, reduced frequency and magnitude of fluctuations are likely to enhance overwinter survival ofage-O Colorado pikeminnow. CONCLUSIONS · There was a relationship between survival index and total length of age-O Colorado pikeminnow in fall. · There was no apparent relationship between survival index of age-O Colorado pikeminnow and flow variability. · There was no apparent relationship between survival index of age-O ColoradQ pikeminnow and water temperature. · There was no apparent relationship between survival index of age-O Colorado pikeminnow and fall densities of non-native fishes in nursery backwaters. · There was a significant relationship between greater average backwater depth and higher survival index of age-O Colorado pikeminnow. Backwaters deeper than 120 cm mean depth had significantly higher survival than shallower backwaters. . RECOMMENDATIONS . Investigate reliability of catch rate data compared to mark-recapture estimates as indices of overwinter survival of age-O Colorado pikeminnow. . Evaluate antecedent flows for shaping deepened nursery backwaters. . Investigate downstream transport of age-O Colorado pikeminnow during early runoff spikes in March. 20