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DRAFT Reasons for Decline Historically, the Colorado-River was characterized as a river with wide seasonal flow fluctuations (Waters 1946). It was known as Rio Colorado, the great Red River of the West. Over 2,700 km (1,687 miles) long, it is cliffbound in canyons throughout much of its course. It drops over 3.2 km (2 miles) in its journey to the Gulf of California, thereby creating some of the most turbulent waters found on earth. At Lee's Ferry, Colorado River streamflows varied from 750 cfs in 1924 to an estimated 300,000 cfs in 1884 (White and Garrett 1988). Few rivers were so laden with silt. Averaging 0.62 percent silt content by volume, it formerly carried more than 100,000 acre-feet (12,000 ha-meter) of soil to the Gulf of California each year. It also is high in mineral salts: carbonates; sulfates; and chlorides of calcium, sodium, and magnesium. The Colorado River has changed dramatically since the turn of the century. More than 20 dams have been constructed on the mainstem and tributaries since 1913. Declines of native fishes directly downstream from reservoirs are, clearly related to colder water temperatures (Vanicek et al. 1.970). Other, more subtle factors include changes in stream nutrients, al~~tered seasonal and daily discharge patterns, and lowered turbidity. Nutrients that once occurred in the rivers now are retained in the phytoplankton and zooplankton populations of reservoirs.- Water from the hypolimnetic layer of deep reservoirs carries far less dissolved materials and fine particulates to fertilize downstream river reaches. Sediments are trapped by reservoirs so that downstream channel bottoms transform from sand to armored cobble and boulder. Channelization below dams has reduced the number and size of backwaters and sloughs that are sought after by Colorado squawfish and other native fishes for nursery and resting areas. The natural cycle of flood and drought is replaced by stable discharges and water levels; seasonal fluctuations are replaced by variable demands for irrigation water or hydroelectric power. This combination of factors effectively eliminated Colorado squawfish and most other native species in 105 km (65.6 miles) of the Green River below Flaming Gorge Dam (Vanicek and Kramer 1969; Vanicek et al. 1910), caused vast biological modification in essentially the entire 389-km (243-mile) reach of the Colorado River mainstem in Marble and Grand canyons below Glen Canyon Dam (Carothers and Minckley 1981), and resulted in the exclusion of most warmwater fishes, both native and introduced,. from long reaches of the Colorado below Davis Dam (Minckley 1979). Specific streamflows and water temperatures are particularly important to the survival of Colorado squawfish larvae. Representative shallow, ephemeral backwater and shoreline habitats in the Green River have been seined from 1979-85 to determine the relative abundance of larval Colorado squawfish (Jones and Tyus 1985x, 1985b; Tyus et al. 1987). The highest relative larval fish abundances were observed in 1979 (233/1,000 m2 [221/1,000 ft2]) and 1980 (271/1,000 m2 [25/1,000 ft2]). During the high flow years 1983 and 1984, larval squawfish recruitment was significantly lower (5-8/1,000 m2 [.5-.14/1,000 ft2]) (Tyus et al. 1987). Jones and Tyus (1985x, 1985b) recommended that abnormally high water levels in the summer and fall be avoided in areas of the Green River that support Colorado squawfish during 15