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1720 D. B. OSMUNDSON ET AL. Ecological Applications Vol. 12, No. 6 Fausch et al. 1988), yet many studies suggest that biotic factors such as food availability may be more important (Cada et al. 1987, Filbert and Hawkins 1995, Hughes 1998, Nislow et al. 1999). With the exception of the flood-pulse concept, wherein main-channel productiv- ity is enhanced by the entrainment of floodplain nu- trients and carbon (Junk et al. 1989, Bayley 1995, Pow- er et al. 1995), few investigations have attempted to link flow effects to availability of food for fishes. To restore native fish populations in regulated rivers, management strategies must be based on an under- standing, not only of the fishes' life-history attributes, but also of the ecosystem that sustains them (Power et al. 1995, Stanford et al. 1996, Wootton et al. 1996). Petts (1991) identified the need for integration of in- terdisciplinary research that approaches the study of regulated rivers on a longitudinal basis and in a hier- archical manner: hydrology and geomorphology to de- scribe the influence of flow regime and sediment sourc- es on river structure, and ecology to link these influ- ences to riverine fauna. However, large-scale riverine processes are inherently difficult to study (Schlosser 1991, Johnson et al. 1995, Peterson and Kwak 1999). With some exceptions, experimental studies are gen- erally not possible (Power et al. 1995, Poff et al. 1997), so correlative empirical approaches are required to dis- cern linkages among flow, sediment, and biota. The Colorado River, in southwestern United States, is one of the most regulated rivers in North America (Fradkin 1981). Nearly all of the pathways by which regulation can alter or degrade native aquatic fauna are represented, supplying diverse challenges for the ap- plied ecologist. Driven by the mandates of the Endan- gered Species Act, resource managers need solutions to ecologically complex problems (Stanford 1994, Holling and Meffe 1996). Herein, we report the results of our investigations concerning, physical habitat and trophic relations in a contiguous portion of the upper Colorado River inhabited by all life stages of Ptych- ocheilus lucius Girard (Colorado pikeminnow), an en- dangered piscivorous cyprinid. For this population, low recruitment and low adult carrying capacity are two primary constraints that limit population viability (Os- mundson and Burnham 1998, Osmundson et al. 1998). In addition to the obvious effects of reduced range from instream barriers, we hypothesized that the capacity of the system to support adult pikeminnow may have de- clined from a reduction in food availability due to in- creased persistence of fine sediment (particles <2 mm), a possible effect of river regulation. We used an interdisciplinary approach to investi- gating the role of flow regime in riverine trophic struc- ture, integrating geomorphology with studies of fish community structure and biomass of lower trophic lev- els (Petts 1991, Johnson et al. 1995). We first described the distribution of physical habitat and biota on a lon- gitudinal basis throughout the present and historical range of Colorado pikeminnow in the upper Colorado River mainstem. Then, using empirical relations, we sought links among sediment distribution, standing crops of primary producers, consumers, and the prin- cipal piscivore (Colorado pikeminnow). The relation between flow and sediment movement was next as- sessed through standard geomorphological analyses. Utilizing these empirical relations and historical flow records, we evaluated the potential for reduced predator food availability as affected by functional changes in flow. Our goal was to assess links among flow, sedi- ment, and biota in a large river system and determine if such linkages might provide a food-mediated path- way by which flow regime alteration affects fish as- semblage structure. We discuss the implications for na- tive fish restoration efforts in general and provide man- agement recommendations for conservation of Colo- rado pikeminnow in the upper Colorado River. SPECIES OVERVIEW AND STUDY AREA Colorado pikeminnow The Colorado pikeminnow, a large, long-lived pis- civore, along with three other large-river, warm-water fish species (two chubs, Gila cypha Miller and G. ele- gans Baird and Girard, and one sucker Xyrauchen tex- anus Abbott) endemic to the Colorado River Basin, are classified as endangered under the Endangered Species Act (U.S. Fish and Wildlife Service 2000). All four once occurred throughout the basin, ranging from Wy- oming south to the Gulf of California (Miller 1961). Due primarily to extensive dam construction during the 1930s to 1960s, populations of Colorado pikeminnow were extirpated in the lower sub-basin (downstream of Lee's Ferry) by the 1970s and today occur only up- stream of Glen Canyon Dam, primarily in the states of Utah and Colorado (Minckley and Deacon 1968, Minckley 1973). In the upper sub-basin, the range of this species was reduced early in the century by di- versions built for local irrigation projects, and addi- tionally by the more recent (1960s-1980s) construction of several large dams. Lotic environments were con- verted to lentic habitats, downstream water tempera- tures were reduced by hypolimnetic releases, and mi- gration routes were blocked (Ono et al. 1983). After a 75% basin-wide loss of range, a viable population oc- curs today only in the Green River system (Gilpin 1993). A smaller, less viable population occurs in the mainstem Colorado River and its Gunnison River trib- utary (Osmundson and Burnham 1998). Populations of Ptychocheilus lucius appear to require an extensive length of river with an array of habitat types to meet the changing needs of different life stages. Larvae hatched in gravel-cobble substrates of high-gradient reaches drift 100-200 km downstream to low-gradient reaches where backwaters formed in silt-sand bars pro- vide ideal nursery habitat (Haynes et al. 1984, Tyus and Haines 1991). Insectivory is largely replaced by piscivory during the first year (Vanicek and Kramer