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<br />dominated withdrawal plumes in the forebay.
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<br />Hueftle and Stevens: Test Flood Effects on Lake Powell
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<br />DISCUSSION AND MANAGEMENT
<br />IMPLICATIONS
<br />Given the context of antecedent conditions,
<br />these data demonstrated significant impacts on
<br />reservoir and downstream water quality. The
<br />most influential factors were the magnitude and
<br />composition of the 20WI; followed by the
<br />location, magnitude, timing, and duration of
<br />dam discharges, not necessarily in that order.
<br />Had the test flood not occurred during the
<br />hypolimnetic upwelling, nor the ROW been
<br />used, the penstocks alone could not have
<br />substantially flushed the hypolimnion, The
<br />ability of the penstocks to mix and entrain the
<br />hypolimnion is considerably less under normal
<br />discharge levels. Without large, carefully timed,
<br />and/or bi-level discharges, the opportunity to
<br />release meromictic water may be foregone. In
<br />the reservoir, significant shifts in salinity and
<br />DO gradients were observed nears the penstock
<br />and ROW elevations as far as 100 kIn uplake.
<br />Fresher, more oxygenated water was drawn into
<br />the middle-depths of the forebay from the
<br />epilimnion and 20WI uplake. These more dilute
<br />conditions persisted through 1997. Although of
<br />short duration, the test flood affected Lake
<br />Powell limnology in a fashion that provides
<br />insight into the dramatic shifts in water quality
<br />alluded to in the 1980's historical data set
<br />(Hueftle and Vernieu, 2000).
<br />In the tailwaters, jet valve aeration,
<br />attenuation of primary productivity, and the
<br />trace of seiches and meromictic discharge were
<br />strong signatures of the test flood, though
<br />short-lived. Shannon et aI., Stevens et aI., and
<br />Valdez et aI., 200x, address longer-term aquatic
<br />impacts on downstream resources.
<br />These effects are important to in-lake water
<br />quality and determination of down-river water
<br />quality. Currently, large discharges are likely to
<br />occur only during periods of high lake levels and
<br />high inflows, thus, future high releases will
<br />probably occur during periods of declining
<br />meromixis. Should in-lake hypoxia or meromixis
<br />approach levels of concern, however, the test
<br />flood demonstrated a mechanism for their
<br />downstream release. Hypoxia, not always
<br />associated with meromixis, could be managed
<br />with well-timed ROW releases. Dam operations
<br />could influence the banking or release of ion
<br />concentrations, DO, T"C and other components
<br />that were not examined here, such as biological
<br />components. Carefully timed dam releases could
<br />be used to avert problems with minimal impact
<br />to power production and water storage. For
<br />
<br />example, precise releases at peak upwelling in
<br />February or March would require less discharge
<br />volume to reduce meromixis than at other times
<br />of the year. But uplake and downstream effects
<br />must be considered prior to future actions.
<br />This study of large and multi-level
<br />discharges from GCD has global implications for
<br />future reservoir, discharge and down-river
<br />management opportunities, including future
<br />experimental floods, flow regimes, and other
<br />management options that are pending at Glen
<br />Canyon Dam.
<br />Installation of a selective withdrawal system
<br />(SWS) is an option outlined by the Final EIS
<br />(Stanford and Ward 1996). Its purpose, via
<br />epilimnetic withdrawal, is to wann the Colorado
<br />River to encourage mainstem spawning of
<br />endangered native fish. Such action could
<br />produce unforeseen thermal, chemical and
<br />biological changes above and below GCD. Use of
<br />hypo1imnetic discharge may offset some of these
<br />impacts, and continued investigations could
<br />lead to more informed decisions.
<br />The demonstration of the test flood effects
<br />as well as those observed during the 1980's
<br />spillway discharges alludes to impacts we could
<br />expect from the operation of a SWS. Operational
<br />changes will have limnological impacts, and
<br />informed decisions will require a sound
<br />1imnologicaI foundation for management of
<br />water quality resources. Current knowledge of
<br />the strength, destination and quality of winter
<br />underflows and inflows, strength of meromixis,
<br />antecedent conditions and long-range
<br />considerations will be required for informed
<br />management in the future.
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<br />ACKNOWLEDGEMENTS
<br />We would like to acknowledge D.L. Wegner
<br />and the Glen Canyon Environmental Studies
<br />office for their hard work in conducting the test
<br />flood, which could not have taken place without
<br />them. We thank William Vernieu for assistance
<br />in field collection and design, data management
<br />and boat operations. For assistance in field
<br />collections, we thank to R. Radtke (Upper
<br />Colorado Region, U.S. Bureau of Reclamation)
<br />and K. Berghoff and M.O. Cline (Glen Canyon
<br />National Recreation Area). We thanks L.D.
<br />Garrett, E.C. Hueftle , J.P. Shannon, and M.
<br />Yeatts for editorial comments and
<br />encouragement. We also thank the anonymous
<br />reviewers for comments on the draft. This
<br />research was funded by Glen Canyon
<br />Environmental Studies, Grand Canyon
<br />Monitoring and Research Center, and the
<br />Bureau of Reclamation.
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