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December 2002 FLOW-SEDIMENT EFFECTS ON RIVERINE FISH
<br />coordinated dam releases. Experimental releases of suf-
<br />ficient magnitude could be used to test whether the
<br />flushing function can, be achieved with a short-term
<br />flow spike that would require less water than natural
<br />runoff flows of historic duration.
<br />If factors that currently hamper reproductive success
<br />and recruitment of Colorado pikeminnow in the upper
<br />Colorado River can be identified and remedied, the
<br />system must have the capacity to support an enlarged
<br />adult population. In previous studies, mean body con-
<br />dition of adult pikeminnow was significantly lower in
<br />strata downstream of Westwater Canyon (strata 1-6)
<br />than in strata upstream (strata 7-9), and dispersal of
<br />young adults to upstream strata was interpreted as a
<br />response to downstream food limitations (Osmundson
<br />et al. 1998). Recent (1998-2000) declines in mean body
<br />condition of adults both upstream and downstream, co-
<br />inciding with an increase in adult numbers (USFWS,
<br />unpublished data), suggest food limitations were ex-
<br />perienced riverwide. Two methods to increase carrying
<br />capacity of the system are possible and both will be
<br />needed to maximize size of this population and thereby
<br />improve population viability: (1) extend the current
<br />upstream range to include historic habitat now unused
<br />by pikeminnow (i.e., provide fish passage and perhaps
<br />temperature augmentation where appropriate), and (2)
<br />enhance conditions for lower trophic levels in both off-
<br />channel (sensu Junk et al. 1989) and within-channel
<br />(this study) habitats. A combined approach would al-
<br />low increases in both distribution and density of Col-
<br />orado pikeminnow.
<br />CONCLUSIONS
<br />This study demonstrated a clear link between bed
<br />sediment and biomass of benthic organisms in the up-
<br />per Colorado River. Numbers and biomass of fish cor-
<br />responded with biomass of detritus, periphyton, and
<br />invertebrates, strongly suggesting that their numbers
<br />are limited by available food. The downstream decline
<br />in body condition of the dominant native fish species
<br />was consistent with this hypothesis. In this system, with
<br />large annual inputs of fine sediment, flows of sufficient
<br />magnitude are frequently required to winnow silt and
<br />sand from the bed and transport it downstream where
<br />it can be deposited on floodplains or channel margins.
<br />Peak flows from snowmelt runoff historically provided
<br />the energy needed to flush the bed. However, river reg-
<br />ulation, primarily in the headwaters of the mainstem
<br />and Gunnison rivers, has reduced the magnitude of
<br />these flows during the past 50 yr and thereby reduced
<br />the frequency of flushing events.
<br />The link between fine sediment and biomass of
<br />aquatic organisms, although demonstrated here via spa-
<br />tial empirical relationships, has an important temporal
<br />implication. Because fine sediment accumulates in the
<br />bed over time, the frequency of flushing flows may be
<br />a key variable influencing main-channel benthic pro-
<br />duction and ultimately the capacity of the river to sup-
<br />1737
<br />port the native piscivore. Hence, our analysis suggests
<br />another pathway by which river regulation may depress
<br />native fish assemblages in large rivers, and in this case,
<br />place an additional constraint on population viability
<br />of an endangered species. The "natural-flow-regime
<br />paradigm" (Stanford et al. 1996, Poff et al. 1997) may
<br />prove a timely guide for managers of the upper Col-
<br />orado River system. Our results support the concept
<br />that reestablishing functional roles of the natural flow
<br />regime are necessary if efforts to restore native fish
<br />communities in this and other regulated rivers are to
<br />succeed.
<br />ACKNOWLEDGMENTS
<br />We thank the numerous individuals that assisted in data
<br />collection and sample analyses for this project; principal
<br />among these were Bruce Bonar, Bob Burdick, Tim Corda,
<br />Cheri Cornell, Dax Dugan, Margaret Franseen, Dak Hovey,
<br />Lex Ivey, Elizabeth Kline, Keith Lawrence, David Lewis,
<br />Katrina Lund, Chuck McAda, Marty Miller, Ralph Mitchell,
<br />Mike Montagne, Kay Moser, Toby Mourning, Jennifer Nis-
<br />senbaum, Leisa Philips, Rebecca Thomas, Mike Tucker, Jean-
<br />ine Rossa, and Dale Ryden. We also thank Thomas Lisle,
<br />Robert Millions, Mary Power, Charles Rabeni, and two anon-
<br />ymous reviewers for providing valuable comments on earlier
<br />drafts of the manuscript. Frank Pfeifer of the Colorado River
<br />Fishery Project provided logistical support and helped pro-
<br />cure funding; we are grateful for his efforts and encourage-
<br />ment. Funding of three separate studies and DFS monitoring
<br />was provided by the Recovery Implementation Program (RIP)
<br />for Endangered Fish Species in the Upper Colorado River
<br />Basin, a joint effort of federal and state resource agencies,
<br />upper basin water and power user groups, and environmental
<br />organizations. Integration of study results presented here was
<br />funded by the U.S. Fish and Wildlife Service.
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