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EXPERIMENTAL FLOOD EFFECTS ON THE LIMNOLOGY OF LAKE POWELL RESERVOIR , SOUTHWESTERN USA. Susan J. Hueftle1 and Lawrence E. Stevens~ lGrand Canyon Monitoring and Research Center 2255 N. Gemini Dr., Flagstaff, Arizona 86001-1600 (520) 556-7460 and 2Consulting Ecologist P.O. Box 1315, Flagstaff, Arizona 86002 Original research submitted 21 February 2000 to Ecological Applications Accepted for publication, in press Running Head: HueftIe and Stevens: Test Flood Effects on Lake Powell ABSTRACT: 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 hypoIimnion of Lake Powell reservoir and its tailwaters. Prior to the test flood, a six-year drought had produced a pronounced meromictic hypoIimnion 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 Ian3 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 hypoIimnion as far as 100 Ian uptake. This effect was reinforced by seasonal upwelling of hypolimnetic water at the dam and seasonal hydrologic patterns uptake. 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 dielfluctuations 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 Iimnological events, can be used to manipulate tailwater and reservoir water quality. KEYWORDS: Colorado River, dam operations, experimental flood, hydrodynamics, hypolimnion, hypoxia, Lake Powell, limnology, meromixis, multiple level withdrawal, reservoir, stratification. INTRODUCTION The use of dam operations as a variable to manipulate and experiment with reservoir and riverine systems is in its infancy. It is one element that differentiates reservoirs from natural lakes. In addition, reservoirs differ limnologically from natural takes in their young age, their elongate and dendritic morphology, and because of the diversity of dam design, discharge patterns, and their typically sub- thermocline releases (Ryder 1978, Kennedy et aI. 1982). These characteristics often limit the application of limnological theory derived from natural lakes to reservoirs (Kennedy et al. 1985, Thornton et al. 1990), and the great diversity of pattern and processes in reservoirs, as well as an incomplete state of Irnowledge, has restricted comprehensive predictive modeling of reservoir limnology. This has resulted in an individualistic management strategy for most reservoirs. The use of a large flood release from a dam allows a test of the affects on the limnology and productivity of both the upstream reservoir and the downstream river ecosystems (Ward and Stanford 1983). Thus, large reservoir discharge experiments may be used to improve the general understanding of reservoir limnology as well as refme strategies to improve reservoir management. In this paper we report on the impacts of a large experimental dam release on the limnology of Lake Powell, one of the largest reservoirs in the United States, and the Glen Canyon Dam (GCD) tailwaters downstream. From its conception in the Colorado River Storage Project Act (1956) through 1991, GCD design and operations were motivated by hydroelectric power generation and storage allocations. With the advent of an Environmental Assessment, the Grand Canyon Protection Act (1992) and the Glen Canyon Dam Environmental Impact Statement and Record of Decision (USSR 1995 and 1996, respectively), environmental concerns for the downstream ecosystems were introduced to management policy. While climate and the inflow of the Colorado and San Juan rivers primarily ~ ~ .~. ~ l" j .~ t:;; ..< rl ; , " . ~ , .> ,< ~.~ ~: -:' ,'. ~" . ;....;. :";:' .;i;; ;Ii' )1 '" ~. "