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Hueftle and Stevens: Test Flood Effects on Lake PoweU 6 moved downlake. Isopleths indicate the withdrawal zone extended from 50 to 100 or more Ian up-lake, even accounting for vertical uncertainty produced by localized seiche oscillations (Fig. 4 & 6). Chemical data collected near the dam before and after the test flood show consistent decreases in ionic concentrations by an average 4.4%, demonstrating the refreshment of the forebay, particularly in the upper hypolimnion (Table I). The most pronounced shifts surrounding the test flood occurred near the ROW. This was not unexpected due to the meromictic conditions, the influx of fresher conditions provided by the 20WI, and higher discharge, Surface and bottom samples demonstrated the least change. Calculations of the load of salt ions and DO versus relative discharge from the ROWand penstocks illustrate the disparity in discharge versus meromixis (Table II). Although the ROW only accounted for a third of the flood discharge, they contained 23% higher conductance and 33% less DO than is found at the penstocks. Consequently, the introduction of discharges from the ROW had a disproportionate long-term freshening effect upon the hypolimnion compared with penstock withdrawals. Continued dilution of the hypolimnion was apparent (Fig. 5) following the test flood through 1997. This resulted from another high inflow year and continued high releases from February to June 1997, again, commenced during the upwelling event. Rates of change (in percent change per day) for T"C, SC, and DO were calculated for a given point between each of the interpolated isopleths of the main channel from 28 February to 21 April. These calculations excluded the top 30 m of the lake and included the lower 209 kilometers of the length (those zones affected by seasonal influences). The results indicate the greatest changes occurred between 2-5 April immediately following the test flood (summarized in Fig. 8). The next highest rates of change were observed from March 22nd to April 2nd, during the test flood. These results further substantiate the increased effects of the test flood over normal operations. Withdrawal Zone and Tailwaters Effects During the experiment, wind and discharge conditions contributed to water quality oscillations from the Wahweap forebay station to Lees Ferry (Figs. 3 and 7). Internal seiche oscillations are frequently observed in high temporal resolution data sets during the winter at GCD. These oscillations are most evident when the reservoir's chemocline impinges on the penstock elevation, such as dUring the ascending and descending limbs of hypolimnetic upwelling. Isolated wind events such as those on 20 April 1996 (National Climatic Data Center 1996), initiate wind-induced internal seiche oscillations (Cole 1994, Horne and Goldman 1994, Wetzel 1975) of T"C, SC, DO and pH at the dam. As mentioned previously, changes in release rates also create rapid water quality shifts at the penstock level due to the strength and dimensions of the withdrawal plume (Hart and Sherman 1996, Hueftle and Vernieu, 2000). Although at least 4 strong wind events (Fig. 3) occurred during the test flood and created complex interfering seiche patterns, the magnitude and timing of oscillations resulting from the test flood are clearly distinguished from wind induced seiches (Fig, 3). The use of the hollow jet valves (the release structure for the ROW) also creates a unique signature. The valves ejected four plumes of aerated water 10 m above the tailwater pool. Combined with the draft tube discharges from the penstocks, the higher discharge was more turbulent than nonnal discharges. Turbidity and total suspended solids increased from 0.2 to 0.6 NTU and 2 to 19 mgfL, respectively, during the test flood (US Geological Survey 1996). The effects of spray and turbulence from the hollow jet valves immediately oxygenated the tailwaters, resulting in mean DO saturation increases from 79% to 105% (Fig. 7). Typically, T"C, DO and pH reflect fluctuating diurnal patterns that develop in the highly productive 25 Ian tailwater stretch of normally clear, lower flows (Angradi et al. 1992, Ayers and McKinney 1996). Respiration of Cladophora glomerata (the dominant algae) and other life forms contribute to diel pH and DO fluctuations, while TOC responds to insolation. During the test flood, diurnal pH patterns were attenuated (Fig. 7), demonstrating the reduction of respiration due to increased drift (Shannon et al. this volume) and lower light availability resulting from higher discharges, greater turbulence, and deeper water (M. Yard and D.L. Wegner, pers. comm.). Diurnal pH and DO fluctuations recovered quickly (within hours) once lower discharges recommenced, although net respiration was reduced from pre-flood levels due to the sheared biomass. Diurnal pH fluctuation levels had returned to pre-flood levels by late April 1996. During the test flows, diurnal DO patterns, though still present, were overshadowed by jet valve aeration. Conductivity reflected short-term seiche effects and higher salinity of the ROW