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<br />0023.c 7
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<br />Running Head: Test Flood Effects on Lake Powell
<br />
<br />DRAFT
<br />
<br />EXPERIMENTAL FLOOD
<br />EFFECTS ON THE LIMNOLOGY
<br />OF LAKE POWELL RESERVOIR,
<br />SOUTHWESTERN USA.
<br />
<br />Susan J. Hueftle
<br />Lawrence E. Stevens
<br />
<br />Glen Canyon Environmental Studies /
<br />Grand Canyon Monitoring and Research Center
<br />Department of Interior
<br />P.O. Box 22459
<br />Flagstaff. Arizona 86002-2459
<br />(520) 556-7460
<br />
<br />Submitted 2/18/98
<br />To Ecological Applications
<br />
<br />ABSTRACT
<br />The 1996 test flood from Glen Canyon Dam affected
<br />Lake Powell reservoir limnology and tailwaters.
<br />Hypolimnetic withdrawals from the penstocks and the river
<br />outlet works (ROW) interacted with ongoing
<br />hydrodynamics and stratification patterns. Over the course
<br />of 9 day spring test flood, 0.626 kIn' were released from
<br />the penstocks and 0.267 kIn' were released from the ROW,
<br />ports located at and below the hypolimnetic chemocline.
<br />This was the largest release since 1986. Prior to the test
<br />flood, a six-year drought had produced a pronounced
<br />meromictic hypolimnion weakened by high inflow events
<br />in 1993 and 1995. Hypoxia, however, continued to increase
<br />in the deepest portions of the reservoir. Over the course of
<br />the experiment and in the weeks and months that followed,
<br />the volume of this hypoxic and meromictic hypolimnion
<br />was diminished as far as 100 kIn uplake. This effect was
<br />coincidentally reinforced by characteristic upwelling of
<br />hypolimnetic water at the dam, as well as by established
<br />seasonal hydrologic patterns. The timing of the test flood
<br />maximized the release of some of the highest salinity and
<br />lowest dissolved oxygen (DO) water that typically occurs
<br />near the release structures of the dam throughout the year.
<br />The increased discharge and mixing induced by the test
<br />flood continued to freshen the hypolimnion for many
<br />months following the even~ reinforced by subsequent high
<br />inflows and discharges in 1997.
<br />During the flood, elevated discharges in the
<br />tail waters briefly increased DO to above saturation but
<br />dampened diel fluctuations in pH and DO. Ion
<br />concentration levels were elevated during the test flood and
<br />resumed a freshening trend following the lower
<br />hydrograph. Our results indicate that dam operations, timed
<br />with predictable limnological events, can manipulate
<br />tail water and reservoir water quality.
<br />KEYWORDS: Colorado River, dam operations,
<br />experimental flood, hydrodynamics, hypolimnion, hypoxia,
<br />Lake Powell, limnology, meromixis, multiple level
<br />withdrawal, reservoir, stratification.
<br />
<br />INTRODUCTION
<br />Large releases from dams may affect the limnology
<br />and productivity of both the upstream reservoir and the
<br />downstream river ecosystems (Ward and Stanford 1983).
<br />Reservoirs are limnologicalIy different from natural lakes,
<br />because of their young ages, their elongate and dendritic
<br />morphology, and because of the diversity of dam design
<br />and discharge patterns (Ryder 1978, Kennedy 1982). These
<br />characteristics often limit the application of limnological
<br />theory derived from natural lakes to reservoirs (Kennedy et
<br />al. 1985, Thornton el al. 1990), and the great diversity of
<br />pattern and processes in reservoirs, as well as an
<br />incomplete state of knowledge, has restricted
<br />comprehensive predictive modeling of reservoir limnology
<br />and change through time. This has resulted in an
<br />individualistic management strategy for most reservoirs.
<br />Reservoirs may be subjected to large-scale withdrawal
<br />experiments that elucidate basic limnological processes and
<br />relationships, which often cannot be tested in natural lakes.
<br />Thus, large reservoir discharge experiments may be used to
<br />improve the general understanding of reservoir limnology
<br />as well as refine strategies to improve reservoir
<br />management. In this paper we report on the impacts of a
<br />large experimental dam release on the limnology of Lake
<br />Powell, one of the largest reservoirs in the United States,
<br />and the Glen Canyon Dam (GCD) tailwaters downstream.
<br />From its conception in the Colorado River Storage
<br />Project Act (1956) through 1991, GCD design and
<br />operations were motivated by hydroelectric power
<br />generation and storage allocations. With the advent of an
<br />Environmental Assessment, the Grand Canyon Protection
<br />Act (1992) and the Glen Canyon Dam Environmental
<br />Impact Statement and Record of Decision (USBR 1995 and
<br />1996, respectively), environmental concerns for the
<br />downstream ecosystems were introduced to management
<br />policy. The stratification and hydrodynamics of Lake
<br />Powell are primarily influenced by climate and the inflow
<br />of the Colorado and San Juan rivers, but dam design and
<br />operations strongly influence the routing and discharge
<br />rates of various limnological strata within the reservoir and
<br />consequently, reservoir water quality (Hart and Sherm~
<br />1995, Hueftle and Vemieu 1998). Therefore, although
<br />Colorado River ecosystem management has not been
<br />guided by concerns for Lake Powell linmology, dam
<br />discharges have influenced the limnology of the reservoir
<br />and shoreline ecology of this large reservoir (Potter and
<br />Drake 1989), as well as the regulated river ecosystem
<br />downstream (Stevens el al. 1997, Valdez el al., this issue).
<br />Several features of dam design influence
<br />limnological development of Lake Powell. The location of
<br />the penstocks, the primary withdrawal port in GCD, has
<br />affected stratification patterns. The penstocks are located at
<br />a mid-depth bordering on the hypolimnion! epilimnion
<br />boundary, and draw from the hypolimnion :5 half of the
<br />year. By isolating the hypolimnion from direct discharge,
<br />meromixis (stagnation and chemical concentration)
<br />frequently occurs. Meromixis is characterized by relatively
<br />high specific conductance (a measure of salinity) and an
<br />upper boundary defined by a chemocline (chemical
<br />gradient) which resists mixing. Hypolimnetic stagnation
<br />and high DO demand can result in hypoxia or anoxia.
<br />Anoxia and the associated reducing environment can
<br />produce hazardous compoWlds, such as hydrogen sulfide,
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