Experimental flood effects on the limnology of Lake Powell reservoir, southwestern USA

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 La ke Powell reservoir and its tailwaters. Prior to the test flood, a six-year drought had produced a pronounced meromictic hypolimnion that was weakenin g 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 t he test flood, 0.893 km(3) were released from the ports located at and belo w the hypolimnetic chemocline. The increased discharge and mixing resulting from the test flood diminished the volume of this hypoxic and meromictic h ypolimnion 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 in flows and discharges in 1997 continued to freshen the hypolimnion. During the flood, large aerated discharges in the tailwaters briefly increa sed DO to above saturation but dampened diel fluctuations in pH and DO. Dow nstream ion concentration levels were elevated during the test flood but re sumed an enhanced freshening trend following the lower hydrograph. The resu lts indicate that dam operations, timed with predictable limnological event s, can be used to manipulate tailwater and reservoir water quality.