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22 BIOLOGICAL REPORT 24 0 a 0 A K ???--- i i t i i x. O 8 l ! 1 K i -'?J/ O L Time -.? Fig. 15. Models of the dynamic relationship between native and nonnative fishes in regulated (A) and unregulated (B) arid-land streams. A: In a regulated stream native fishes (solid line) typically decline and disappear after introduction (x) of nonnative fishes (dashed line). B: In a free-flooding stream, native fishes similarly decline after nonnatives appear, but flooding (arrows) reduces nonnative populations to levels that permit recovery of native fishes. During interflood periods, population size and range of nonnative fishes again expand and negatively impact native species until the next flood. If flooding occurs frequently enough, long-term coexistence may occur as a dynamic equilibrium. K = carrying capacity of the stream for native fishes (from Minckley and Meffe 1987). major disturbance events, like flooding, may not occur the same way 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 down- stream, is that the rhithron-potamon transition zone is pushed downstream, producing an ecologi- cal discontinuity (sensu Ward and Stanford 1983). Biophysical conditions characteristic of headwa- ter (rhithron) segments occur in reaches that were characterized by warmwater conditions before regulation. Very productive coldwater food webs, including stenotherms such as stoneflies and trout (Fig. 1), establish in waters that were inhab- ited by potamon species prior to impoundment. Regulation of the Gunnison River by the Aspi- nall Units (Fig. 9) has produced a classic and well documented ecological discontinuity. The position of the rhithron-potamon transition has shifted downstream 70-80 km (Ward and Stanford 1991) as a consequence of reduced peak flows and colder water temperatures. 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., Fig. 11) owing to hydropower operations, and the hypolimnial releases have cooled the river at the confluence of the North Fork (Fig. 1) by nearly 10° C during summer (Stanford and Ward 1983). A reproducing (wild) rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) fishery (Nehring 1988) devel- oped in association with a biodiverse and very productive coldwater zoobenthos community from Crystal Dam through the Gunnison Gorge to be- low the confluence of the North Fork (Fig. 16; Hauer et al. 1989; Stanford and Ward 1989; Ward and Stanford 1990, 1991; Stanford and Ward 1992b). Hence, the rhithron-potamon transition zone, which occurred within the Gorge prior to regulation, now occurs below the North Fork con- fluence. Creation of this substantial ecological discontinuity, coupled with construction of the Redlands and Hartland diversion dams, which blocked migration pathways many years ago (Quartarone 1993), undoubtedly has contributed to the demise of squawfish and razorback sucker in the Gunnison River, where they were formerly abundant (Tyus 1984; Minckley et al. 1991; Tyus 1991a). The new rhithron community in the regulated Gunnison River, however, is extremely fragile ow- ing to the responsiveness of the ecological discon- tinuity to flow and temperature, as controlled by reservoir releases (Stanford 1989). Indeed, the