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Authors: PRocEEDtrvcs oFTru DES’RT FISISS CoUNCIL, 1995 Sylaposruu VoIIlltG)Ofi/II - PIJEISIS 1996

Year: 1995 (xxvii)

Abstract

We examined subadult (less than 20O mm TL) humpback chub Gth rypha densities along i5 miles of the Colorado River in Grand Canyon to identify relationships between geomo.pholog’, hatitat use and Glen Canyon Dam florv operations. We categorized shorelin“ n“Uit”t basfd or, g“ootpholory. We then measured physical habitat Parameters of velocity, depth and cover along all shoreline types over the range of Interim Flow Operations (B,0OO to 20,000 cfs) to determine if habitat changes with discharge. . Although cover, depth and velocity were significantly related to subadult humpback chui presence, they oplained very little of the overall variation in subadult humpback chub densities. This variance may !e partially erylained by a significant relationship of discharg” i habitat quality. We found that cover decreased with increasing discharge, whereas depth and rr.t“Aty increased- These results suggest that habitat quality decreases with increasing discharge. Historically, basg florvs ranged from l,OO0 to 5,000 ds. Both preferred alternatives of the Glen Canyon Dam Environmental Impact Statement require base flows .gi.tg between 8,0O0 and 20,000, nearly anorder of magnitude higher than pre-dam base flows. Impacts from the reduction of peak flows have beenthoroughly assessed in regulated rivers; however, “..ording to our results, artificially high base flows mayactually limit habitat quality in Grand Canyon. t”“ro,-“pApERcoMpETrroRI 29