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r structure that can be correlated to longitudinal changes in habitat and other physical parameters (Ames 1977). Many of the changes in community structure among stations can be directly or indirectly attributed to the flow regime at each site. It is likely that flow (primarily velocity and depth) exerts a major influence on macroinvertebrate community structure in the Yampa River by: 1) imposing physical, behavioral or life history constraints on species in the community, 2) by influencing biological interactions such as regulating the availability of food supply, and influencing algal species composition and rate of growth, and 3) by controlling and limiting the availability of specific habitat. Native Fishes Colorado Pikeminnow Colorado pikeminnow are piscivorous, long-lived, large-river fish that utilize a variety of substrates, depths, and velocities. Juveniles up to 50 mm (2 in) in length consume zooplankton and insect larvae. Colorado pikemllnow from 50-100 mm (2-4 in) in length feed on insects, and individuals larger than 200 mm (7.9 in) are primarily piscivorous (Vanicek and Kramer 1969). During spring and early summer, adult fish use areas inundated by spring flooding. Colorado pikeminnow usually become sexually mature within 5-7 years (Vanicek and Kramer 1969). Sexually mature Colorado pikeminnow can migrate long distances (up to 322 km [200 miles]). This behavior is important to this species' reproductive cycle. Adult Colorado pikeminnow are known to migrate long distances for spawning (Tyus and McAda 1984; Wick et al. 1983; Wick et al. 1986; Tyus 1990). Tyus (1990) notes that physical conditions such as high spring flows and increasing river temperatures are important migratory cues. He reported that Colorado pikeminnow spawning migrations were initiated at water temperatures of 14-20oC (57.2-680F), while spawning occurred at an average temperature of 220C (71.60F). Colorado pikeminnow demonstrate a fidelity to spawning locations (Tyus 1985; Tyus 1990; Wick et al. 1983; Irving and Modde 1999), with reproduction occurring in whitewater canyons. Once larval pikeminnow emerge, they undergo a period of drift to reach suitable nursery habitats. During the larval drift, they may be transported up to 161 km (100 miles) downstream (Tyus and Haines 1991). Nursery areas consist of ephemeral backwaters and shoreline embayments with little or no current (Tyus and Haines 1991). Wick et al. (1983) reported migration from the Upper Yampa to the Lower 32km (19.9 mi) of the Yampa Canyon during late June and early July. Tyus (1990) reported migrations occurring between late May and mid June on the Yampa River and late May and late June on the Green River. Miller et al. (1982) reported migrations between late June to early July. Osmundson and Kaeding (1989) reported increased movement of Colorado pikeminnow from late June and early July to late August on the Colorado River near Grand Junction, Colorado. 29