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01190 RIVER RESE:\RCH AND APPLICATIONS Rirer Res. A.pplic. 20: 379--400 (2004) Published online 10 February 200,", in Wiley lmerScience (www.interscience.wi\ey.com). 001: IO.IOO2/rra.749 MODELLING EFFECTS OF DISCHARGE ON HABITAT QUALITY AND DISPERSAL OF JUVENILE HUMPBACK CHUB (GILA CYPHA) IN THE COLORADO RIVER, GRAND CANYON JOSH KORMAN.'" STEPHEN M. WIELEb and MARGARET TORlZZO' . Ecomerrir: Researrh Inc.. 3560 W 22nd A~'e., VmICollvt'T, Be. V6S 113. Cannda b Warer Rc'sourCt's Dh'isioll, us Geological Sun:e>; 520 N. Park An'" Suite 221. TUC.s01J, AZ 857/9, USA C Waler Resourres Divisioll. US Geological SU1ifey. Denver Federal Center. Denver. CO 85225. USA ABSTRACT A two-dimensional hydrodynamic model was applied to seven study reaches in rhe Colorado River within Grand Canyon to examine' how operation of Glen Canyon Dam has affected availability of suitable shoreline habitat and dispersal of juvenile humpback chub (Gila cypha). Suitable shoreline habitat typically declined with increasing discharges abo.....e 226-425 m.1/s. although the response vaned among modelled reaches and was strongly dependent on local morphology. The area of suitable shoreline habiti.lt over cover Iypes that ure preferred by juvenile humpback chub, however, stayed conslant, and in some reaches. aClUJI]y increased with disch.1Ige. In generiJl. chan2es in discharge caused by impoundment tended to deereuse availability of suitabk shoreline habiwt rrom September to February. but increased habitat availability in spring (May-June). Hourly variation in discharge from Glen Canyon Dam subslilntially reduced the amount of persistent shoreline habitat at a\l reaches. Changes in suitable shoreline habitat with discharge were sho\....n to potentially bias historical catch per unit effort indices of native fish abundance up to fourfold. Physical ret~n[ion of randomly pJ<lced particJes simuJatjng !he movement of juvenile humpbuck chub in the study reaches tended [0 decline with increasing discharge, but the pattern varied considerably due to differences in the locul morphology among reilches and rhe rype of s\~'imming behaviour modelled. Implications of these results to current hypotheses about the cf(ects of Glen Canyon Dam on juvenile humpback chub survival in the mainstem Colorado Ri\"er are discussed, Copyright C!d 201J4 John Wiley & Sons. ltd, ny \\OROS. inSlream flow; humpback chub; Grand Can}"on; Colorado River; suilable habitat; dispersal; hydrodynamic modelling; Glen Canyon Dam INTRODUCTION The Colorado River in Grand Canyon was impounded by Glen Canyon Dam (GCD) in 1963, Since that time. much effort has been expended to assess the downstream effects on aquatic and terrestrial resources (Valdez and Carothers, 1998). Although the range of ecological issues being evaluated is diverse. the most critical set driving potential changes to the operation of the dam relale to humpback chub (Gila cypha). The humpback chub is a large. warm-water. cyprinid fish endemic to the Colorado River drainage that was listed as endangered in 1967 and protected under the Endangered Species Act (ESA) in 1973. One element of the Reasonable and Prudent Alter- native (RPA) of the US Fish and Wildlife Service's (USFS) Biological Opinion on the operation ofGCD called for the experimental implementation of a seasonally adjusted steady flow (SASF) consisting of sustained high releases during the period of April-May. followed by low steady releases during June-October (US Fish and Wildlife Service, 1994), The low steady summer flow (LSSF) experiment conducted in the spring and summer of 2000. at an estimated cost of approximately $21 million in additionally purchased power (Palmer and Burbidge, 2001). was the first implementation of the SASF. . Correspondence to: J. Korman, Ecomeuic Research Inc., 3560 West 22nd A\'enue, Vancouver, BC, V6S In, Canada. E.mail: jkorman@ecometric.com Copyright ild 2004 John Wiley & Sons, lid, Received 9 Jllly 2002 Revised /9 November 2002 Accepled /0 December 2002