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<br />HuefUe and. Stevens: Test Flood Effeds on Lake PoweU
<br />
<br />Feny, 25 kIn below the dam. These measured
<br />TOC, SC, DO, and pH at half-hour intelVals,
<br />However, the high flows of the test flood
<br />rendered some of this information unusable.
<br />The Hydrolab profIles provided the fmest
<br />resolution and the most consistent data sets--
<br />particularly at the greater depths affected by the
<br />penstock and ROW withdrawals.
<br />All Hydroiab instruments were calibrated
<br />using standard solutions and established
<br />protocol (Hydrolab 1994) before and after each
<br />sampling period. Blanks, duplicates, and spiked
<br />sam pies were collected for every 10 chemical
<br />samples.
<br />
<br />Analyses
<br />Data were compiled, reviewed and analyzed
<br />using SAS and Lotus software. Grapher (Golden
<br />Software 1994), and Surfer (Golden Software, V.
<br />6.04, 1996) software were used to generate two
<br />and three dimensional (isopleth) graphics,
<br />respectively. Isopleths illustrating lake-wide
<br />hydrodynamic processes plot various
<br />parameters against depth (in elevation) and
<br />river channel distance uplake from Glen Canyon
<br />Dam. Long-term trend analysis was facilitated
<br />by temporal isopleths plotting various
<br />parameters against depth and time. An
<br />animation sequence of the lake-wide
<br />conductivity isopleths since 1965 is available at
<br />www.usbr.l!ovllzces.This demonstrates
<br />hydrodynamics, underflows and discharges of
<br />the reselVoir including profIles of the test flood.
<br />
<br />RESULTS
<br />
<br />Discharge Hydrograph and Lake Elevation
<br />Prior to the test flood, the dam had
<br />discharged at above average levels since June
<br />1995 as a result of large inflows that spring.
<br />Flows were increased from 280-340 m3 j s to
<br />480-537 m3js in June and maintained there
<br />until October 1995, and thereafter averaged
<br />340-425 m3js until the test flood in 1996.
<br />On 26 March 1996, penstock and ROW
<br />releases were increased to 850 m3js and
<br />425 m3js, respectively (Fig, 3), A total volume of
<br />0.893 kIn3 was discharged during the test flood;
<br />0.626 kIn3 from the penstocks and 0.267 kIn3
<br />from the ROW. Following the experiment,
<br />discharges from the dam were increased to high
<br />fluctuating levels of 450-566 m3 j s for the
<br />duration of the spring to accommodate the large
<br />1996 snowpack. Although the test flood is
<br />identified by the 7 days of high releases, the
<br />experiment included 8 days of low steady flows
<br />bracketing the flood, (Patten et al., this volume)
<br />
<br />4
<br />
<br />which also produced effects to lake and
<br />tailwaters.
<br />The test flood directly affected lake
<br />elevation. Over the course of the experiment,
<br />between March 220d and April 8th, reselVoir
<br />elevation had a net drop of 0.98 m. Although the
<br />reselVoir dropped 1.12 m during the test flood,
<br />the 4 days of 227 m3 j s discharges preceding
<br />and following the 1,274 m3js flood increased
<br />reselVoir stage by 0.15 m, The lake elevation
<br />changes were slightly more than anticipated
<br />because of the later onset of the high spring
<br />inflows. Soon after the experiment concluded,
<br />the reselVoir elevation increased substantially.
<br />The sudden drop in lake elevation required that
<br />water stored in the more eutrophic side-bays
<br />enter the mainstem (Thornton et al. 1990). The
<br />data suggests mainstem nutrient levels may
<br />have increased throughout the reselVoir in June
<br />1996, accompanied by increased chIorophyll-a
<br />and -c and pheophytin-a. However, the existing
<br />IWQP includes few side-bay collections,
<br />particularly in the lower reach. Therefore,
<br />trends from side-bays are not conclusive and
<br />cannot be verified, but suggest further
<br />investigation and imply management
<br />considerations,
<br />
<br />Stratification and Hydrodynamics: Antecedent
<br />Conditions
<br />The previous decade's climate and inflow
<br />patterns affected the limnological conditions
<br />prior to the test flood, and understanding these
<br />is critical to interpreting the results of the test
<br />flood on reselVoir stratification and
<br />hydrodynamics. From 1987 to 1994, Lake
<br />Powell's drainage basin experienced extended
<br />drought; 6 of those years were among Lake
<br />Powell's lowest inflows in the reselVoir's 33-year
<br />history. This resulted in a pronounced
<br />monimo1imnion with a pycnocline (density
<br />gradient) resistant to mixing. This stratification
<br />was weakened by 2 high inflows (5th and 6th
<br />highest in the lake's history) in 1993 and 1995.
<br />These inflows introduced a large pool of lower
<br />SC water for winter mixing in the epilimnion.
<br />Numerous authors, including Merritt and
<br />Johnson 1978, Johnson and Merritt 1979,
<br />Gloss et al. 1980, Gloss et al, 1981, Edinger et
<br />al. 1984, and Stanford and Ward, 1986 and
<br />1991, have described Lake Powell's density
<br />currents. Normal winter hypolimnetic processes
<br />are dominated by partitioned underflows that
<br />form in the inflows and migrate advectively
<br />toward the dam (Hueftle and Vemieu, 2000).
<br />The fIrst winter underflow (IOWU) forms in the
<br />fall as a relatively warm, saline mass of dense
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