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<br />Hueft1e and Stevens: Test Flood EffeclS on Lake Powell <br /> <br />5 <br /> <br />~ <br />'.; <br />ij <br />;.' <br />:'1 <br />~~. <br />tl <br />" <br />\-; <br /> <br />water flows along the fonner riverbed toward the <br />dam, dispersing through and thickening the <br />monimolimnion. The secondary winter <br />underflow (2'WU) fonns in the inflow at the <br />peak of winter, a cold, convectively mixed ~~ss <br />of relatively cold, oxygenated and lower salimty <br />water which follows the l'WU down-lake. <br />Although its density is rarely sufficient to <br />completely displace the hypolimnion, the 2'WU <br />may refresh the stagnant hypolimnion if it is of <br />sufficient magnitude and density, and dam <br />discharges are favorable. Most commonly, this <br />2'WU reaches the chemocIine midway down the <br />thalweg in the reservoir and becomes an <br />interflow (2'WI) , overriding or passing through <br />the hypolimnion, depending on its relative <br />density. It is then drawn into the penstock <br />withdrawal zone. This 2'WI occurs regularly, <br />and its freshening potential increases with the <br />depth the density cur:ent. achieves. before <br />diversion over the hypolimmon. Precedmg the <br />test flood, the 2'W underflow was in transition <br />to a 2'W interflow 135 Ian uplake. These <br />conditions were similar to those in 1994 <br />(155 Ian) and 1995 (110 Ian) (Fig. 4). <br />A second component of the freshening <br />2'WU is the advective force it applies to the <br />hypo1imnion. While rarely able to penetrate the <br />chemocIine, the advective forces of the 2'WI are <br />often sufficient to depress the hypolimnion, <br />creating a periodic .upwelling" of the <br />hypo1imnion. As a result, the chemocIine <br />ascends the face of the dam for a period of <br />weeks to months. This effect can be seen in the <br />3-year forebay isopleths (Fig. 5), with the <br />upwelling effect typically beginning in February, <br />peaking in March, and diminishing by May. <br />Prior to the test flood, upwelling had already <br />peaked by mid-February and was subsiding. <br />The upwelling effect is diminished: 1) by <br />discharge through the dam, and, 2) subsidence <br />of the upwelling as the advective forces of the <br />2'WI dissipate. The animation sequence as well <br />as the synoptic channel profiles (Fig. 4) <br />demonstrate annual winter upwelling cycles <br />evident near the dam. <br />The upwelling pattern maxlIJuzes <br />hypolimnetic discharge through the penstocks <br />and ROW. However, the interflow pattern can <br />confuse the interpretation of test flood impacts <br />with seasonal hydrodynamics already <br />underway. By late 1995, the 2'WU had shifted <br />to a 2'WI and its descent along the thalweg of <br />, . <br />the lake slowed as it impinged on the pycnocline <br />and diverted horizontally downIake toward the <br />penstocks. From the onset of the test floo~, <br />inflow hydrodynamics actively affected reservOIr <br />limnology at the penstock elevation. Therefore, <br /> <br />distinguishing test flood effects from existing <br />seasonal change required an examination of <br />rates of change on water quality and the impacts <br />from the ROW. <br /> <br />Effects on Stratification and Hydrodynamics <br />Test flood effects on Lake Powell were <br />observed through shifts in chemoclines with <br />consequent changes in str~ta volume, ~d <br />through shifts in water quality. The synoptic <br />channel promes (Figs. 4 and 6) and temporal <br />Wahweap forebay isopleths (Fig. 5) demonstrate <br />the descending migration of the chernocline and <br />DO gradients during the test flood. Comparisons <br />with the previous year's upwelling and <br />subsidence patterns show the test flood effects <br />were most pronounced at the ROW depth, where <br />the freshening effects of the 2'WI discharge were <br />most dramatic. Prior to the test flood, 3 <br />distinctive strata were distinguished from SC <br />and DO concentrations at the Wahweap forebay <br />station (Figs. 4, 6): 1) an upper convectively <br />and wind mixed epi1imnion underlain by a <br />distinct chemocIine 7.5 m above the penstock <br />outlets; 2) a 24 m-thick 2'W~ middle layer <br />underlain by a second chemocIine 13 m above <br />the ROW; and 3) a lower 66 m-thick <br />monimoIimnion. Changing the elevation and <br />magnitude of discharge restructured these <br />layers. As a general rule, increases in discharge <br />result in a 3'" power increase in kinetic energy <br />available for mixing, as KEDDDQ3 (Thornton et <br />aL 1990); this extends the vertical draw of the <br />outlets (Monismith et aI. 1988). Hence, the <br />increase from the normal penstock discharges of <br />390 m'ls to bi-Ievel discharges of 850 m'ls and <br />420 m'l s from penstocks and RO~ increas~d <br />mixing energy by an order of magmtude, while <br />total discharge only increased 3-fold. <br />The addition of sub-hypolimnetic discharge <br />intensified vertical mixing. With the onset of the <br />bi-Ievel high releases, the upper chemocline <br />weakened as the penstocks drew more heavily <br />from the epilimnion and the 2'WI. Profile data at <br />the dam demonstrated refreshment at the <br />penstocks as they drew from the epi1imnion (Fig. <br />3). But below the dam at Lees Ferry, co-mingled <br />penstock and ROW release~ show an. overall <br />increase in ionic concentrations, reflecting the <br />dominance of ROW hypolimnetic output (Fig. 7). <br />The chemocline below the 2'WI and between the <br />outlet ports weakened and descended more than <br />12 m to the level of the ROW at the conclusion <br />of the flood. The 2'WI stratum was thickened <br />16 m as it drew from the wider wedge uplake, <br />entraining the epi1imnion and hypolimnion and <br />weakening the associated chemoclines as it <br /> <br />I <br /> <br />7., <br /> <br />;t", <br /> <br />r~'J <br />~..: <br />3-i. <br />"" <br />5' <br />. 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