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.... 358 Computarional Fluid Dynamics Th~ Colorado River corridor in Grand Canyon National Park has be~n sev~rely alt~red from its free-l1owing state by the construction and closure of Glen Canyon Dam. The onc~ turbid. sodiment-Iaden !low is now clear ~xcept during tributary !lows. Th~ current av~rage annual sand volume in transport below th~ tirst major ,.dim~nt-contributing tributary b~low the dam. th~ Paria River. is about 1.5 million metric tons. or about 6% of the pre-dam sedim~ntload (Topping el 01.. 20ooa). Pre- darn average annual flood p~aks of 2420m.1js hav~ been typically reduc~d to the poy,~rplant capacity of 930 m'ls ~xc~pt on sewral occasions during exceptional hydrologic conditions or. as of this writing. a single experimental rel~ase (see Webb elal.. 1999). Winter base flows have been incr~ased from less than about 80m.'/s prior to the dam construction to about 140mJjs under curr~nt dam operations. Th~s~ chang~s to the !low and sedim~nt characteristics of the river have had dir~ct impacts on d~pendent resourc~s. such as the preservation of archaeological art~facts. riparian vegetation and endangered species such as th~ humpback chub and the Kanab Amber snail. The retention of the main-stem sand supply behind Glen Canyon Pam also has led to the erosion of downstr~am sand. deposits that fonn the substrate for riparian flora and fauna. are used as campsites by riverside visitors to th~ national park and are an important aesthetic attribute of the natural, formerly unregulat~d river in the park. The only tool curr~ntly und~r consid~ration for mitigating the effects of Glen Canyon Pam on the downstream river corridor is the darn itself. An Environm~ntal Impact Study (EIS; CS Department of Interior. 1995) proposed releasing water at discharges in excess of powerplant capacity to ~ntrain and suspend sediment stored at lower elevations within the chann~1 for redeposition at higher elevations along the channel sides. An experimental release from Glen Canyon Dam in 1996 demon- strated that the dam can be operated to restore d~posits along the channel sides if sufficient sand is available. The experimental release consisted of a ..eady discharge of 225m3/s for 4 days rising to a st~ady 1270m'js for 7 days. followed by ano[h~r sl~ady 225 mJls for about 3 1/2 days. As summarized by Schmidt(1999), many of the objoctives for a high release contained in an EIS (liS Department of Interior. 1995) were achiewd by the 1996 release. River guides report~d improved camping condi- tions after the 1996 test flow (Thompson elal., 1997). Kearsley ela/. (1999) reported an increase in usable campsites from 218 to 299. Studies of a~rial photos (Schmidt elal.. 1999) and bathymetric measurem~nts before, during and after the test flow (Andrews elol.. 1999) documented significant deposition at many sites. along with consid~rable spatial and temporal variability. Although many sandbars were replen- ish~d. sandbars closest to the dam. and therefore with the smallest available sand supply, were more likely to erode (Schmidt, 1999). More recent studies, moreover. have concluded that the channel sand-storage capacity is much less than was posited in the EIS (Topping el al.. 2000b) and have suggested that th~ timing of high 110ws should be adjusted to follow shortly after tributary inputs (Lucchitta and Leopold. 1999; Rubin elal.. 2(02) to maximize the use of the available sediment. Studi~s of the g~omorphology of near-river environments wh~r~ archaeological sit~s are locat~d (Hereford etal.. 1991. 1993) have concluded that the lower peak discharges and lower sand concentrations sinc~ the c1osur~ of GI~n Canyon Pam hav~ dir~ctly or indirectly damaged som~ of those sites. H~reford NO/. (1991. 1993) concluded that the bigh discharges released from the dam during 1983 directly - .' I f. . - .