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Page 20 ~ Basin, nearly 12,000 within the Bear River drainage, and over 21,000 within the entire Green River Basin in Utah. These are the most extreme basins in terms of number of diversions and most three species drainages have only 1000 to 5000 diversions (Utah Division of Water Rights6). It is important to note that not all of these diversions are substantial; however, even the ~ smallest diversions can impact native fish through entrainment and fragmentation, and when taken together, the cumulative impacts can potentially be severe. Most streams in the Colorado River Basin provide irrigation water for hundreds of thousands of acres of crops and only small amounts of water for municipal and industrial uses. Moving north, however, into the Bonneville Basin and the Bear and Weber River systems, which support limited bluehead sucker populations, the situation is quite different. Hydrology of the Bonneville Basin streams is largely regulated by mainstem impoundments intended for water allocation purposes and flood control. The Bear River has three power dams, 13 storage/diversion dams, and over 50 diversions of various sizes and various uses from its ~ headwaters, north into Wyoming and Idaho, and south to its endpoint at the Great Salt Lake in Utah. The Weber River does not have as many diversions; however, it is heavily regulated by water development projects, large interbasin transfers, and three storage reservoirs. Though more water from this basin is used for agricultural than municipal uses, the population of this basin is significantly higher than those found in the less populated Colorado River Basin streams. ~ Timing of these diversions can also be problematic. In Utah, surface flow is generally at its lowest in July and August, which is often when diversions are most needed. Timing issues such as these, can contribute to the de-watering of streams, such as happened in 2004 in Muddy Creek (Walker and Birdsey 2005). The de-watering of this stream section likely "prevented . downstream escapement of resident fish and it is unlikely that these fish survived the ...dewatering" (Walker and Birdsey 2005). In order to prevent this scenario, Walker and Birdsey (2005) suggest coordinating water use in drainages where de-watering has recently been documented to reduce the potential for isolating upstream stretches. Nonnative predators and competitors ~ The Colorado River has a predominant endemic fish population (42 of 49 species; of the total 49, only 14 were found in the upper basin) that is very specifically adapted to a harsh and highly variable environment (Hoetker and Gobalet 1999, Minckley et al. 1986, Smith .1981). Speciation and diversification in the Colorado River Basin was very limited in comparison with the eastern United States where hundreds of species of fishes can be found in each state (Starnes and Etnier ~ 1986). Because Speciation was so limited, these fishes never had to adapt to large numbers of predators and competitors. Thus, they have had a great deal of difficulty adjusting to the purposeful (i.e., for sportfishing purposes) and accidental (e:g., baitfish and biological control) introduction of at least 72 nonnative species of fishes throughout the basin (Behnke 1980, Minckley et al. 1986, Ono et al. 1983). These introductions have negatively impacted the native ~ fishes of the Colorado River Basin (Carlson and Muth 1989, Lentsch et al. 1996, Martinez et al. 1994, Minckley 1991, Tyus and Saunders 1996, Valdez and Carothers 1998) through predation (Douglas and Marsh 1998, Nesler 1995, Ruppert et al. 1993, Tyus and Nikirk 1990), competition and trophic interactions (Lamarra 1999, Osmundson 1999), introduction of nonnative pathogens (Bezzerides and Bestgen 2002, Heckmann et al. 1993,Robinson et al. 1998b,) and hybridization S (Douglas and Douglas 2000, Holden and Stalnaker 1974). Some researchers have suggested that 6 http://www.waterrights.utah.~v/pisinfo/. Utah Division of Water Rights GIS website.