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01 " The prediction that flooding will limit predation mortality of endangered fishes. is used as one rationale in the recovery program for reinstatement of peak flows. I note that introduced species, red shiners for example, are native in rivers that experience floods (of bankfull or greater) rather frequently, which might suggest that flow augmentation might not work very well in , . controlling some nonnative species. However, the complex interactions described above that are associated with major disturbance events, like flooding, may not manifest the same in all rivers or all river reaches, even if they are prone to flooding. The relationship needs to be examined and compared in constrained and unconstrained reaches. Stream Regulation in an Ecosystem Context: Occurrence of Ecological Discontinuities The cumulative effect of regulation, especially when deep release dams control the flow downstream, is that the rhithron-potamon transition zone is pushed downstream, producing an ecological discontinuity (sensu Ward and Stanford 1983). Biophysical conditions characteristic of headwater (rhithron) ,segments manifest in reaches that were characterized by warm water conditions before regulation. Very productive cold water food webs, including stenothenns such as stoneflies and trout (Figure 1), establish in waters that were inhabited by potamon species prior to impoundment Regulation of the Gunnison River by the Aspinall Units (Figure 9) has produced a classic and well documented ecological discontinuity. The position of the rhithron-potamon transition has shifted downstream 40 - 50 miles (Ward and Stanford 1991) as a consequence of reduced peak flows and colder water temperatures. Indeed, bankfull discharge of 11,000 cfs in the Gunnison Gorge downstream from the dams occurred every 3.2 years before regulation. Given the storage capacity of the Aspinall Units, the historical water yield of the catchment and current regulation regime, bankfull discharge will occur only once in 40 years in the future (Elliott and Parker 1992). Moreover, baseflows are high and variable (e.g., Figure 11) owing to hydropower operations, and the hypolinmial releases have cooled the river at the confluence of the North Fork (Figure 1) by nearly 100C during summer (Stanford and Ward 1983). A reproducing (wild) rainbow and brown 38