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<br />}~~ <br /> <br />~~ <br />',~; <br /> <br />Under CUrren~ operations, Flood a.l..... Will Occur <br />In About on. Of BV.ry Four Y.ar. <br /> <br />< <br />, <br /> <br />:~i <br /> <br />Flood releases occur about one in four years due to <br />reservoir storage targets and errors in forecasted <br />runoff (among other variables), CUrrent data are <br />sufficient to show that this frequency of flooding <br />would De damaginq to downstream resources, but are <br />insufficient to determine precisely the frequency of <br />flooding that resources can tolerate in the long-term. <br />Based on oDservations of the natural system in Grand <br />Canyon, flood releases should De avoided until a <br />tolerable frequency can be better defined. CUrrent <br />knowledge indicates that even a frequency as low as one <br />flood in twenty years will produce a net long-term loss <br />of camping beaches and substrate, although at a rate <br />reduced from that caused by current operations. <br /> <br />i;.1 <br />, <br /> <br />c <br /> <br />~I <br /> <br />~ <br />% <br />~ <br />. ' <br />G <br /> <br />,-. <br />~ <br /> <br />. <br />Two method. of frequency analysis were used to arrive <br />at the one-in-four-year flood frequency. Operating <br />procedures and methods in place during the GCES study <br />period were used in calculating the frequency of <br />spills, <br /> <br />~', <br />~ <br />.~ <br /> <br />:.:.: <br />. ) <br /> <br />~, <br /> <br />Fluctua~inq R.l..... primarily Attect a.cr..tion <br />and Aquatic R.sourc.. <br /> <br />~I <br />',~l <br />~ <br />h~! <br />~'\ <br />o <br />~ <br />~! <br /> <br />Except during periods of very high runoff, the amount <br />of water relea.ed from Glen Canyon Dam is varied on an <br />hourly Dasi., often with two peaks and two troughs <br />daily. Thi. i. done to provide electrical power when <br />it is most needed during the day. These fluctuations <br />can cause the river level to change Dy up to 13 feet. <br />Fluctuating releases stay below 31,500 cfs and are <br />therefore not as detrimental as floods for terrestrial <br />resources. However, they have a deleterious effect on <br />recreation and aquatic resources. The quality of fiSh- <br />ing and White-water boating is reduced by approximately <br />15 percent under fluctuating releases as compared to <br />steady releases. <br /> <br />~,~,' <br />~ <br />~ <br />~ <br />~ <br />~ <br /> <br />Fluctuating releases have a greater impact on <br />than on terrestrial resources. Fluctuations <br /> <br />aquatic <br />at any <br />