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398 J. KORMA!":', S. ~l. WIELE .~;-.."D M. TOR{ZZO fish sampling sites whenever possible to identify the sites where differences in dischJrge between sampling periods shou\d be minimized. OUf simulation of movement and dispersal of young fish in sites downstream from the LCR demonstrates that even moderate swimming abilities result in :1 large improvement in the "ability of fish to move into low- velocity habitat' and to not be d"placed downstream (Figure 10), A \00 mm fish swimming al 2 body lengths/s (Bainbridge. 1958) can maintain a cruising speed of approximately 0,2 mls. consistent with speeds measured by Bulkley el ai, (1982) for humpback chub, In our simulations, this swimming speed typically resulted in retention rates 1.6 (rheotactic) to 2.9 (geotactic) times larger than those for passively drifting particles. This suggests thai discharge is likely to have a significant effect only on retention rates of larvae and small YOY fish entering rela- tively early in Ihe dispersal period (May-June) from the LCR.lfreduced retention leads to decreases in mainstem survival rates, our simulation results provide a mechanism for the hypothesis that larval and smaller YOY hump- back chub (<52 mm TL) dispersing from the LCR do not sur..i\'e'(Valdez and Ryel. 1995; Robinson el al,. 1998), There is great u!lcertainty in the reliability of numerical habitat models to prediclthe responses of fish popula- tions to changes in flow because the responses depend on a complex series of interactions between habitat and ecological processes (Walters and Konnan, 1999). This is especially true in Grand Canyon where exotic species are present in high numbers, Why then. did we pursue our anal)'sis? Habitat models have the potential to provide some guidance about designing flow experiments. For example, if simulation results showed a dramatic improve- ment in suitable shoreline habitat availability at 350 m'ls, it could be argued Ihal this would be a more appropriate discharge for LSSF experiments, Similarly. breakpoints in the relationship between daily discharge variation and the amount of persistent suitable habitat could be used to design experiments testing the effects of fluctuating flows, The potential effect of discharge-driven changes in suitable shoreline habitat availability on CPE data high- lights the importance of considering Ihe interactions between flow treatments and our ability to monitor population responses to these treatments, an issue that is probiJ.bly applicable to adaptive management experiments on other regulated rivers. . AC~"O\VLEDGDIE~.S This project was funded by grants from the GCMRC to the USGS and Ecometric Research, Thanks to: Barbara Ralston (GCMRC) for authorizing funding and her support for this study; Barbara Ralston and Christopher f. Smith for reviewing the initial draft of the manuscript and Zack Bowen and an anonymous reviewer for reviewing Ihe final draft; Jack Schmidt and Paul Grams (Utah State University) for providing electronic versions of Iheir surficial maps; Steve Mietz (GCMRC) for providing G[S data; Bill Persons (Arizona Game and Fish Deptartmenl) and Chris Flaccus (GCMRC) for providing electrofishing data; and David Topping (USGS) for providing .the digitized 1922-[986 Lees Ferry discharge record, REFERENCES Bainbridge R. 1958. The speed Qf .swimming fish as related to sile and to fteq'Jency and amplitude or the tail beat. Journal of E.:cperim.enuzl Biology 35(1): 109-133. Bovee KD. 1982. A Guide to Stream Habitat AMIJsis Using the In.strtam Flow Incremental Methodology. Uniled States Fish and Wildlife Service Biological Services Program, Cooperati...e Instream Flow Service Group. I~m Row Infonnation Paper Numbet 12. FWS/OBS-82-46. Bowen ZK, Freeman Me, Bo...ee KD. 199~. Evaluation of generalized 'habiL"l1 criteria for assessing impacts of altered now ~ginles on Waml~ water fishes. Transactions o/the American F;sheri~s Societv 121; 455-468. Bulkley RV, Beny CR, Pimentel R. Black T. 1982. Tolerance ~nd Pr~ferences of Colorado Endang~red Fishes to S~/ected Habitat Parameters. Colorado River Fishery Project Final Report Part 3. US Fish and Wildlife Se",,'ice. Bureau of Reclamation: Sail Lake City. {IT; 185-241. casulli V. 1990. Semi-implicit finite difference methods for the tv..o-dimernional shallow water wave equations. Journal of Computation(ll Physics 86: 56-74. Converse YK. Hawkins CP, Valdez RA. 1998. Habitat relalionships of subadult humpback chub in [he Colorado River through Grand Canyor1: spatial variability and lrt\{llicatlOI;\S. o{ fl.ow regulations. ReguloreJ Rivers 1~: 261-284. Dolan R. Howard AD. Trimble D. 1978. S[ructural control of rapids and pools of the Colorado River in the Grand Canyon. Science 20t: 629~3t. Environmental Systems Research Institute, Inc. 1991. ARCnNFO User's Guid~-SlJrface Modeling .....ilh TlN™. Environmental Systems. Research lns\ilu\e hc.: Redlands, CA. Copyright (0 2004 John Wiley & Sons, Ltd. Rj~'er Res. Applic. 20: 379-400 (2004)