In the spring of 1996, a nine-day test flood from Glen Canyon Dam involved the deepest and largest hypolimnetic withdrawals from the penstocks and the river outlet works (ROW) since 1986, interacting with ongoing hydrodynamic and stratification patterns to enhance freshening of the hypolimnion of Lake Powell reservoir and its tailwaters. Prior to the test flood, a six-year drought had produced a pronounced meromictic hypolimnion that was weakening from high inflow events in 1993 and 1995. Hypoxia, however, had continued to increase in the deepest portions of the reservoir. Over the course of the test flood, 0.893 km3 were released from the ports located at and below the hypolimnetic chemocline. The increased discharge and mixing resulting from the test flood diminished the volume of this hypoxic and meromictic hypolimnion as far as 100 km uplake. This effect was reinforced by seasonal upwelling of hypolimnetic water at the dam and seasonal hydrologic patterns uplake. The timing and magnitude of the discharge maximized the release of the highest salinity and lowest dissolved oxygen (DO) water that typically occurs near the release structures of the dam annually. Subsequent high inflows and discharges in 1997 continued to freshen the hypolimnion.
During the flood, large aerated discharges in the tailwaters briefly increased DO to above saturation but dampened diel fluctuations in pH and DO. Downstream ion concentration levels were elevated during the test flood but resumed an enhanced freshening trend following the lower hydrograph. The results indicate that dam operations, timed with predictable limnological events, can be used to manipulate tailwater and reservoir water quality.
