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<br /> <br />114. <br />37"-f--- <br />I ~\~I.' <br />, '. <br />~,,1~ ~. <br />\ <br />I <br /> <br />Graf <br /> <br />UTAH 1130 <br />--ARIZONA-T------ <br />~ <br />". <br />o <br /> <br />Grand <br />Canyon <br /> <br />I ......" <br />\ <br /> <br />.,.. <br />';;' <br />'" <br />" <br /> <br />3S. <br /> <br /> <br />" <br /> <br /> <br />Kingman <br /> <br />BlI68 from U.S. GeologIcal Survey, <br />1 :500.000, Slale base map <br /> <br />GAGING STATleN <br />AND NUMBER <br /> <br />o <br />I <br />o <br /> <br />50 MILES <br />I <br /> <br />",09404200 <br /> <br />EXPLANATION <br /> <br />I <br />50 KILOMETERS <br /> <br />Gneiss Canyon 0 DYE. SAMPLING SITE <br />6 SUBREACH NUMBER <br /> <br />Figure 1. Study Area and Location of Dye-Sampling Sites and Subreachel!J. <br /> <br />Releases from Glen Canyon Dam can range from <br />about 30 to about 850 m3ts, Daily mean releases have <br />been higher than the powerplant capacity of about <br />850 m3ts only about 3 percent of the time since 1963 <br />(V,S, Department of the Interior, 1994), Typically, <br />water is released from the dam in response to power <br />demand, and resulting releases are high in the middle <br />of the day and low at night, The time-varying dam <br />releases produce discharges that vary rapidly with <br />time - are unsteady - in the reach downstream from <br />the dam (Figure 2), Median daily range in releases <br />has been between 340 and 450 m3ts since flow regula- <br />tion began in 1963, Rate of change of releases was <br />between about 110 and 230 m3ts per hour about 60 <br />percent of the time from 1966-1989 (V,S, Department <br />of the Interior, 1994), The National Park Service, V,S, <br />Fish and Wildlife Service, and Arizona Game and <br />Fish Department, which manage resources down- <br />stream from the dam, and the public have become <br />concerned that unsteady dam releases are adversely <br /> <br />WATER RESOURCES BULLETIN <br /> <br />affecting the riparian environment, The concerns and <br />potential adverse effects have been summarized in a <br />final report of the first phase of the GCES (V,S, <br />Department of the Interior, 1989) and in a review <br />report by a committee of the Water Science and Tech- <br />nology Board of the National Research Council <br />(National Research Council, 1987), <br />As a part of the ongoing environmental studies, the <br />Western Area Power Administration (V,S, Depart- <br />ment of Energy) and the BOR have released water in <br />such a way as to provide opportunities for data coHec- <br />tion that otherwise would not be possible, In October <br />1989, a steady discharge of 142 m3ts was released for <br />a period offour days to provide researchers the oppor- <br />tunity to study low flows, Reach-average velocity and <br />longitudinal dispersion in the reach from Glen <br />Canyon Dam to Lees Ferry (Figure 1) were measured <br />during that steady-release period, Later, researchers <br />and managing agencies agreed to release water to <br />provide a series of "research flows" from July 1990 <br /> <br />266 <br />