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c..,. ChonnoI CoN> 0 A< 8 I ~ mouth SlIJk_ ~ C ! .! - ! ~ D.. N ! . Roundtall Chub eom<Io _ fD ~ ~_4 ~ - - I! m 8. i"ll ai E ",b "II n ~ 1 0 . a '" - v tI:> ~ _TIoul , b m "'"II ~ drift Sol ~ ~b ~'- il~ '" - HdenWy mlgnltory Spatial Fidelity :'.' I i.~1 ~ ;". ~. ,f t' f) .'d' t~~. I ~:. ,.' " '.' ! h. . ~:. i. ~~;:; ~.... i ~, j!"ft i..~;..' ...., L ,. L:. ; ~ . Figure 4. Guild box of spatial fidelity, feeding strategy, and temperature preference for the eight native fish species and 11 principal non~native fishes of the Colorado River in the Grand Canyon. Extirpated native species are shown in red, endangered native species in blue, and nonwnative species in green. Circles indicate location with respect to spatial fidelity and feeding strategy, and venical placement of fish symbols indicates temperature preference. ,. ~~. . ...... . 1996). Different rypes of sandbars vary in their susceptibility to erosion (Schmidt er al. 1995); erosion may be greatest in the narrowest parts of the Grand Canyon (Schmidt and Graf 1990, Kearsley et al. 1994). Biological processes. Fish assem- blages native to the Colorado River evolved in an environment of highly variable discharge, large annual tem- perature fluctuation, high turbidity, large input of organic material, and the opportunity for basinwide fish migration (Valdez and Ryel 1997). These conditions have all changed. For example, annual river tempera- rures before the construction of Glen Canyon Dam ranged berween ap- proximately 0 and 29 'C. River tem- perature no longer varies seasonally; it is now determined by the tempera- ture of the reservoir at the level at which warer is withdrawn into pen- September 1998 stocks that lead to rhe power plant. The penstocks are at a fixed eleva- tion of 1058 m above MSL (mean sea level), so the deprh of withdrawal depends on how much water is in the reservoir. The reservoir is full at 1128 m above MSL. In most summers, water comes from beneath a thermal discontinuity -jess than 20 m below the surface. These waters have a rela- tively constant temperature of be- tween 8 and 10 'Co Isorhermal con- ditions prevail in winter, when the reservoir sarface is lowest. The heat content of the reservoir is highest in early autumn, when temperatures from the surface to depths of be- tween 15 and 20 m are as high as 30 'C (Stanford and Ward 1991). These seasonal changes in the reservoir's thermal regime provide the opportu- nity to increase summer river tem- peratures by decreasing the depth from which water is withdrawn through the construction of multi- level intake structures on the pen- stock intakes. Water quality in the Grand Can- yon is controlled primarily by pro- cesses in Lake Powell, although pho- tosynthesis by benthic algae and submerged aquatic vegetation deter- mines daily changes in oxygen con- cenrration and pH in the 25 km of the river immediately downstream from the dam (Marzolf et al. 1996); tributary contributions of sedimen and salt affect water qualiry farther downstream (Taylor et al. 1996). Because water chemistry in Lake Powell is controlled by temperature, the chemistry of water released from the dam depends on the region in rhe reservoir from which the water is drawn. Warm surface waters haveJ lower concentrations of nutrients (carbon, nitrogen, and phosphorus) and salrs, whereas cold water, which : . ,. h,' t.",-, I'.. j. .. f'.,c'. , " ~,. " ;j.~ f.J.~ i"7.:~:. , j;.,..l' .. i. \ :.,..' 739