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4 <br />Spanish Bottom <br />Spanish Bot <br />i was selected as a study site because it was above <br />the upper rapid of Cataract Canyon and below the confluence of the Green <br />and Colorado River . The river is narrow and a relatively straight run <br />for 5.6 km from Sp nish Bottom to the confluence (Figure 2). A large <br />back eddy is prese t where the river turns south at the lower end of <br />Spanish Bottom. S all back eddies are present along either bank. Nets <br />were fished in back eddies or along the shore in slower reaches. <br />Below the conlfluence the gradient of the river just above the first <br />rapid averages ap roximately 0.2 m/km. Gradient then increases to <br />approximately 3 m km (Baars and Molenaar, 1971). Most of the drop in <br />Cataract Canyon o curs in relatively short rapids, in which the rate of <br />fall may exceed 5.7 m/km (30 ft/mi). When Glen Canyon Reservoir (Lake <br />Powell) is comple ely full at the 1,128 m level, all but 23 rapids in <br />19.3 km are innun ated. Rapid 23, or Big Drop, is the most severe rapid <br />in the canyon, th rate of fall is 7 m/km (38 ft/mi) from river mile <br />203.5 to the star of the reservoir (Baars and Molenaar, 1971). <br />Gypsum Canyon <br />Gypsum Canyo was selected as a study site because it was just <br />below the last ra id in Cataract Canyon, had a relatively large expanse <br />of calm water, an provided good locations for gillnet sets. <br />During 1980 when river flow exceeded 1,416 m3/sec (50,000 cfs) <br />the mixing zone f the river and reservoir extended for several miles <br />below the last rapid. The headwaters where the Colorado River entered <br />Glen Canyon Reserjvoir resembled a large, swift river at the initial