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<br />- <br /> <br />trout fishery (Nehring 1988) developed in association with a biodiverse and very productive cold <br />water zoo benthos community from Crystal Dam through the Gunnison Gorge to below the <br />confluence of the North Fork (Figure 16) (Hauer et al. 1989, Stanford and Ward 19~~, Ward and <br />Stanford 1990, 1991, Stanford and Ward 1992b). Hence, the rhithron-potamon transition zone, <br />which occurred within the Gorge prior to regulation, now occurs below the North Fork confluence. <br />Creation of this substantial ecological discontinuity, coupled with construction of the Redlands and <br />Hartland diversion dams which blocked migration pathways many years ago (Quaterone 1993), <br />undoubtedly has contributed to the demise of squawfish and razorback sucker in the Gunnison <br />River where they were formerly abundant (Tyus 1984, Minckley et al. 1991, Tyus 1991a). <br />However, the new rhithron community in the regulated Gunnison River is extremely fragile <br />owing to the responsiveness of the ecological discontinuity to flow and temperature as controlled by <br />reservoir releases (Stanford 1989). Indeed, the new rhithron food web, including the valuable trout <br />fishery, was severely damaged by the episodic side flows (described above) that occurred during the <br />summer of 1991-92, when the regulated flows were at or near the 300 cfs minimum. Benthos and <br />fish were smothered by fine sediments, a situation which has persisted owing to the lack of a spring <br />flush to clean the substratum (my observation). Recent experimental flows to help detennine flow <br />recommendations for endangered fishes in the Gunnison River reached 4,000 cfs in 1992 but were <br />insufficient to rearrange alluvium entrained in the river channel (Elliott and Parker 1992). Due to <br />the interactive effects of 1) a lack of spring peaks or other flushing flows, 2) an extended period of <br />minimum flow (both 1 and 2 due to drought and regulation), 3) wanner temperatures associated <br />with low flows and 4) episodic loading of the channel from ephemeral side flows, the position of <br />the discontinuity moved upstream during 1991-92, and side channels and eddies filled in with fine <br />sediments and vegetation. Today the riparian corridor of the river is densely vegetated, and thus, <br />surface and groundwater exchange with critically important backwater systems (e.g., Figure 2) has <br />been altered or lost (Stanford and Ward 1992b). The food web in the lower part of the Gunnison <br /> <br />39 <br />