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<br />Final Report <br /> <br />1-6 <br /> <br />September 2000 <br /> <br />flows in an unregulated river. Seasonal variation decreases as a result of water storage and <br />intentional limitation of releases to accommodate power-generation capacities. Daily variation <br />increases as hydroelectric generation fluctuates in response to daily demands for electricity. Changes <br />in natural seasonal and daily flow patterns affect riverine ecology in a number of ways. Lower spring <br />flows are less effective than higher flows in transporting sediment, forming and rehabilitating <br />important in-channel habitats, scouring encroaching riparian vegetation, and maintaining habitat <br />complexity. In unregulated rivers, overbank flooding in spring during wetter years inundates <br />floodplain wetlands that supply nutrients and organic material to the main channel and serve as <br />important growth and conditioning habitats for some native fishes. Connections between these warm, <br />productive habitats and the river are eliminated or reduced at lower spring discharges resulting from <br />typical dam operations. Higher summer flows limit the formation of backwaters, used as primary <br />nursery areas by many nati ve fishes, and they reduce water temperatures. Higher winter flows flood <br />low-velocity habitats and can potentially displace and stress fish. Daily fluctuations in flow may <br />affect the stability and productivity of nearshore quiet-water habitats (e.g., backwaters). <br /> <br />Effects of regulation on river flow, sediment load, and temperature diminish at increasing <br />distances downstream of dams. Daily flow fluctuations are naturally attenuated by longitudinal <br />changes in channel morphology, and river water warms naturally as it is exposed to sunlight and <br />heated land surfaces along the channel margin. Tributary inputs also ameliorate effects of dams on <br />flow, sediment, and temperature. Tributaries can contribute significant quantities of sediment to the <br />main channel and replenish sediment deposits, particularly during peak-flow events. Tributaries also <br />can restore a more natural seasonal pattern of flow, reduce daily fluctuations produced by <br />hydropower operations, and increase water temperatures. The relative influence of tributaries <br />depends on the size of their drainages, their sediment-load characteristics, and the degree of river <br />regulation. <br /> <br />The natural ecology of rivers also has been affected by widespread increases in nonnative <br />fishes that were intentionally (to create or support sport fisheries) or accidentally introduced into the <br />basin. Reductions in the distribution and abundance of native fishes in the Colorado River basin have <br />been attributed partially to the establishment of more than 60 nonnative fishes (Carlson and Muth <br />1989). Introduced species vary in body size, environmental tolerances, and habitat preferences and <br />are widely distributed and abundant. For example, more than 95% of small-bodied fishes found in <br />Green River backwaters occupied by native fishes in early life stages are nonnative cyprinids (Haines <br />and Tyus 1990; McAda et a1. 1994a; Bestgen and Crist 1999; Day et a1. 1999a, 1999b; Trammell and <br />Chart 1999). Because of their wide distribution, high abundance, and diets ranging from herbivory <br />to piscivory (Tyus and Karp 1989; Tyus and Karp 1991; Muth and Snyder 1995), introduced fishes <br />are potential competitors with or predators of native fishes in nearly all life stages. Although both <br />cold-water (e.g., rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta) and warm-water <br />species (e.g., red shiner Cyprinella lutrensis, common carp Cyprinus carpio, channel catfish <br />Ictalurus punctatus, green sunfish Lepomis cyanellus, and small mouth bass Micropterus dolomieui) <br />can adversely affect native fishes (Hawkins and Nesler 1991; Lentsch et a1. 1996b; Tyus and <br />Saunders 1996), warm-water species have the greatest potential impact because their habitat <br />preferences are similar to those of most native species. <br />