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<br />Hueftle and St.euens: Test Flood Effects on Lake Powell <br /> <br />2 <br /> <br />influence the stratification and hydrodynamics <br />of Lake Powell, dam design and operations <br />strongly influence the routing and discharge <br />rates of various limnological strata within the <br />reservoir and, consequently, reservoir water <br />quality (Hart and Sherman 1996, Hueftle and <br />Vernieu, 2000). Although Colorado River <br />ecosystem management has not been guided by <br />concerns for Lake Powell's limnology, dam <br />discharges have influenced the limnology of this <br />large reservoir (Potter and Drake 1989). as well <br />as the regulated river ecosystem downstream <br />(Stevens et al. 1997, Valdez et al., this volume), <br />Several features of dam design influence <br />limnological development of Lake Powell. The <br />location of the penstocks, the primary <br />withdrawal port in GCD, has affected <br />stratification patterns. The penstocks are <br />located at a mid-depth bordering on the <br />hypolimnionl epilimnion boundary, and draw <br />from the hypolimnion almost half of the year. By <br />isolating the hypolimnion from direct discharge, <br />meromixis (stagnation and high chemical <br />concentration) frequently occurs. Periods of <br />meromixis are characterized by relatively high <br />hypolimnetic specific conductance (a measure of <br />salinity) and an upper boundary deCmed by a <br />chemocline (chemical gradient) resistant to <br />mixing. <br />Hypolimnetic stagnation and high dissolved <br />oxygen (DO) demand can also result in hypoxia <br />or anoxia. Anoxia and the associated reducing <br />environment can produce hazardous <br />compounds, such as hydrogen sulfide, which <br />may pose hazards in-lake and downstream to <br />both living organisms and to metal surfaces, <br />such as the powerplant turbines. Drought <br />conditions have resulted in several episodes of <br />pronounced meromixis in Lake Powell since <br />1963, including the years preceding the test <br />flood. Likewise, stagnation and DO demand has <br />produced hypolimnetic hypoxia as low as 1.4 mg <br />DO I L near the dam. Extremely low DO <br />concentrations have not yet reached discharge <br />elevations; a minimum of 4.5 mg DOlL has <br />been recorded at the penstock elevation. <br />The river outlet works (ROW) are located <br />deeper in the hypolimnion, almost always in the <br />zone of meromictic stagnation. They are seldom <br />used since they bypass power generation, but <br />their location and operation would affect <br />meromixis as they draw entirely from the <br />hypolimnion except during the lowest lake <br />stage. Data suggest that higher flow-through, <br />and ROW withdrawals may diminish the extent <br />of hypolimnetic meromixis. <br />The existence and operation of Glen Canyon <br />Dam (GCD) has significantly altered post-dam <br /> <br />water quality in Glen and Grand Canyon <br />(Hueftle and Vernieu, 2000, Stevens et al. 1997). <br />The presence and operation of the dam has <br />greatly dampened seasonal variations in river <br />flow; temperature, turbidity and ionic <br />concentration variability has been reduced to <br />uniformly cold, clear, low nutrient waters. Post- <br />dam discharge patterns have fluctuated greatly <br />on a daily and weekly basis in response to <br />power demands and are currently constrained <br />by set ramping rates. Water quality and <br />discharge below the dam is now dictated by <br />reservoir water quality and the dam operations <br />(Stanford and Ward 1986, 1991, Angradi et al. <br />1992). Interactions between the magnitude, <br />duration, frequency, timing, and location or <br />discharges from the dam influence uplake water <br />quality which, in turn, determine downstream <br />water quality (Hueftle and Vemieu, 2000). The <br />effect of the unusually large and deep <br />withdrawals of the test flood occurs in the <br />context of seasonal limnological processes, <br />obscuring cause and effect relationships. <br />However, historical data allow comparisons of <br />similar antecedent conditions without <br />corresponding large discharges. <br />The 1996 test flood provided an opportunity <br />to quantify these effects and elucidate the <br />linkage between reservoir and downstream <br />water quality. In this paper we address the <br />following objectives: (I) describe the historical <br />development of Lake Powell limnology; (2) <br />determine whether the test flood's larger <br />penstock discharges and releases from alternate <br />structures affect Lake Powell limnology; (3) <br />determine the extent of discharge required to <br />produce measurable effects and how far uplake <br />such effects are detectable; and (4) determine <br />the impacts on down-river water quality. The <br />large historical database (1964 through 1997), <br />and the large size of this reservoir allow better <br />comprehensive analysis of test flood effects. <br />Analysis of the limnological changes associated <br />with a single, large discharge event may <br />contribute to improved management of this and <br />other large reservoirs that develop meromixis or <br />hypoxia, in addition to improving the linkage to <br />downstream water quality. <br /> <br />METHODS <br /> <br />Study Area <br />Glen Canyon Dam was completed in 1963, <br />part of a series of dams resulting from the 1922 <br />Colorado River Compact and the 1956 Colorado <br />River Storage Act, providing for allocation and <br />storage of water across the arid Colorado River <br />basin, GCD is a 216.4 m high arch construction <br />