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ECOLOGY AND CONSERVATION OF NATIVE FISHES IN THE UPPER COLORADO RIVER BASIN 175 Nonnative Fish Nonnative fishes curtently dominate the ichthyo- fauna of the Colorado Rivet system, and certain spe- cies have been implicated in reductions of native fish populations (Carlson and Muth 1989). At least 67 species of nonnative fish have been introduced into the Colorado River system during the last 100 years (Tyus et al. 1982; Carlson and Muth 1989; Minckley 1991; Minckley and Deacon 1991; Lentsch et al. 1996; Pacey and Marsh 1998). About 50 are found in the upper basin (Table 2; e.g.,Tyus et al. 1982; Lentsch et al. 1996). Many of these spe- cies were intentionally introduced as game or for- age fish and others inadvertently gained access with game fish stockings and baitfish releases. Channel catfish were introduced into the upper basin in 1892 (T yus and Nikirk 1990) and are now wide- spread and common to abundant with docu- mented predation on native species (Tyus et al. 1982; Hawkins and Nesler 1991; Nelson et al. 1995; Lentsch et al. 1996; T yus and Saunders 19%; Brooks et al. 2000; Chart and Lentsch 2000). Northern pike escaped from Elkhead Res- ervoir into the Yampa River in the early 1980s and have expanded into the middle Green River (Wick et al. 1985; Tyus and Karp 1989; Tyus and Beard 1990; Hawkins and Nesler 1991; Nesler 1995). Smallmouth bass also escaped from upstream res- ervoirs and riverside ponds, and have increased in distribution in the Yampa, Green, and upper Colo- rado rivers. Negative effects of nonnative fishes are princi- pally predation, competition, antagonistic behavior, and vectors of parasites and diseases (Karp and Tyus 1990b; Hawkins and Nesler 1991; Tyus 1991; Muth and Nesler 1993; Muth and Snyder 1995; Lentsch et al. 1996; T yus and Saunders 1996; Bestgen 1997; Bestgen et al. 1997; Holden 1999; McAda and Ryel1999; Valdez et al. 1999). Red shiner, common carp, fathead minnow, channel catfish, northern pike, green sunfish, black bullhead, and largemouth bass were of greatest concern be- cause of suspected or documented negative inter- actions with native fish (Osmundson 1987; Hawkins and Nesler 1991; Ruppert et al. 1993; Lentsch et al. 1996). Recent increases in distribu- tion and abundance of smallmouth bass and north- ern pike have raised concern over the impact of these predators on native fish communities and launched more aggressive management efforts. White sucker have also increased in numbers in the upper basin and incidence of hybridization with native suckers may be increasing. Degraded Water Quality Water quality in the upper basin has been substan- tially altered since the late 1800s. Land-use prac- tices have increased sedimentation; agricultural re- turns have increased concentrations of pesticides and herbicides and other elements through leaching; water diversion has reduced the dilution capacity of the river; and dams have trapped elements and nu- trients in reservoirs and changed element concen- trations and water temperatures below outlets. The greatest changes in water quality have occurred in tailwaters below main-stem dams, where there is a measurable reduction in seasonal variability of streamflow and temperature, increase in daily stream fluctuation, reduction in sediment load, and in- creased nutrient and ionic concentrations (Carlson and Muth 1989; Muth et al. 2000). For example, before Flaming Gorge Dam on the Green River, river temperature ranged from summer highs of 280C to persistent ice cover in winter (Vanicek et al. 1970). After the dam, hypolimnetic releases have ranged from 4-130C (Muth et al. 2000). High spring snowmelt flows and spurious, in- tense late-summer rainstorms within a sparsely veg- etated and arid basin have historically produced high sediment loads and low water clarity. Historically, suspended sediment was highest during three dis- tinct periods. Spring runoff produced a consistent period of moderate sediment from late February through June; summer Quly-September) rainstorms produced short, spurious, and sometimes high sedi- ment loads; and midwinter rainstorms or intermittent snowmelt produced minor peaks in suspended sedi- ment. Before Glen Canyon Dam, average sediment load at Lees Ferry (i.e., total upper basin load) was about 140 million tons per year (range, 50-500 million tons); average postdam load is about 15 million tons per year, or a reduction of89% (Cole and Kubly 1976). Sedi- ments once carried by the Colorado River are now deposited in lake Powell, and in 1986 ranged in depth