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Ecological Applications, 12(6), 2002, pp. 1719-1739
<br />© 2002 by the Ecological Society of America
<br />FLOW-SEDIMENT-BIOTA RELATIONS: IMPLICATIONS FOR RIVER
<br />REGULATION EFFECTS ON NATIVE FISH ABUNDANCE
<br />D. B. OSMUNDSON,1 R. J. RYEL,Z V. L. LAMARRA,3 AND J. PITLICK4
<br />'U.S. Fish and Wildlife Service, Colorado River Fishery Project, 764 Horizon Drive, Building B,
<br />Grand Junction, Colorado 81506-3946 USA
<br />2Department of Rangeland Resources, Utah State University, Logan, Utah 84341-5230 USA
<br />'Ecosystem Research Institute, 975 South State Highway, Logan, Utah 84321 USA
<br />"Department of Geography, University of Colorado, Box 260, Boulder, Colorado 80309-0260 USA
<br />Abstract. Alteration of natural flow regimes by river regulation affects fish distribution
<br />and assemblage structure, but causative pathways are not always direct and may go un-
<br />recognized. The Colorado River population of the endangered Colorado pikeminnow, Ptych-
<br />ocheilus lucius, suffers from low rates of recruitment and reduced carrying capacity. We
<br />hypothesized that availability of prey fish for this large-bodied native piscivore may, in
<br />part, be limited by reduced standing crops of periphyton and macroinvertebrates resulting
<br />from accumulation of fine sediment in the riverbed. We stratified the 373-km-long study
<br />area into 11 strata and sampled various physical and biological parameters in runs and
<br />riffles of three randomly selected 1- to 3-km-long study reaches in each stratum during
<br />base flows of spring and fall 1994-1995. Significant correlations were found between
<br />biomass of both chlorophyll a and macroinvertebrates and various physical metrics that
<br />described the degree of fine sediment accumulation in gravel-cobble substrates. Riffles
<br />were relatively free of fine sediment throughout the study area, but substrates of runs
<br />contained progressively more fine sediments with distance downstream. There was a cor-
<br />responding longitudinal change in biota along the river continuum with greatest biomass
<br />of fish, invertebrates, and periphyton upstream. Adult pikeminnow were concentrated in
<br />upstream strata where potential prey fishes were most abundant. We suggest that fine-
<br />sediment effects on biota have increased in recent years as a result of river regulation.
<br />Historically, spring snowmelt frequently produced flows with magnitudes sufficient to mo-
<br />bilize the bed and winnow silt and sand from coarse substrates. Following regulation, the
<br />mean recurrence interval of such flows lengthened from 1.3-2.7 yr (depending on the
<br />stratum) to 2.7-13.5 yr, extending the duration of fine sediment accumulation and potentially
<br />depressing biotic production. Our results describe and help explain the spatial distribution
<br />of the Colorado River fish community and establish a link between flow, sediment, and the
<br />riverine food web supporting the community's top predator. To maintain intact native fish
<br />communities in this and other river basins, managers need to identify functional aspects
<br />of the natural hydrograph and incorporate these findings into river restoration efforts.
<br />Keywords: benthic macroinvertebrates; Colorado pikeminnow; Colorado River; fish distribution;
<br />flow regime; foodweb; interdisciplinary research; natural-flow-regime paradigm; Ptychocheilus lucius;
<br />river regulation; river restoration; sediment.
<br />INTRODUCTION
<br />Regulation of rivers and streams throughout North
<br />America during the twentieth century altered lotic eco-
<br />systems and contributed to the decline of many native
<br />fish populations (Petts 1984, Minckley and Douglas
<br />1991, Hesse and Mestl 1993, Ligon et al. 1995, Naiman
<br />et al. 1995, Barinaga 1996, Poff et al. 1997). Often,
<br />the causes of rapid fish declines from dams and diver-
<br />sions were direct and readily apparent, e.g., drying of
<br />streams from water diversions, blockage of salmonid
<br />spawning runs by dams, or cold-water releases from
<br />dams into once warm-water fish habitats., However, al-
<br />teration of flow regimes may affect fish indirectly and
<br />causal links to population health are often less appar-
<br />Manuscript received 4 May 2001; revised 8 February 2002;
<br />accepted 11 March 2002.
<br />ent. Nevertheless, the ultimate effects on fish popula-
<br />tions may be profound (Wootton et al. 1996). Synthesis
<br />of accumulated evidence has recently led to the for-
<br />mulation of the "natural-flow-regime paradigm"
<br />wherein recreation of natural-hydrograph functionality
<br />is emphasized in foodweb and native fish restorative
<br />efforts (e.g., Power et al. 1996, Stanford et al. 1996,
<br />Poff et al. 1997).
<br />Flow regime is an important factor influencing fish
<br />distribution and assemblage structure (Stevens and
<br />Miller 1983, Meffe 1984, Bain et al. 1988, Poff and
<br />Allen 1995, Mion et al. 1998, Petersen and Kwak
<br />1999), and its modification affects aquatic biota at the
<br />population and community levels (Schlosser 1991,
<br />Marchetti and Moyle 2001). Past attempts to relate flow
<br />to fish production have been based largely on fish pref-
<br />erences for physical habitat variables (Bovee 1986,
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