WSPC12535 ‎(2)‎

• File Number: 8220.101.10
• Description: Colorado River Water Projects - Glen Canyon Dam-Lake Powell - Adaptive Management
• State: CO
• Basin: Colorado Mainstem
• Date: 2/18/1998
• Author: Susan J Hueftle - Lawrence E Stevens
• Title: Experimental Flood Effects on the Limnology of Lake Powell Reservoir - Southwestern USA - Draft - 02-18-98
• Water Supply Pro - Doc Type: Report/Study

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Draft: Test Flood Effects on Lake Powell

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interfering wave patterns, the magnitude and timing of
oscillations resulting from the test flood is clearly
distinguished from wind induced seiches (Fig. 3). Whether
resulting from a seiche or dam operations, SC patterns, in
particular, are translated downstream, and may create a
traceable chemistry signature which may be used for
tracking parcels of water, potentially avoiding more
invasive or costly approaches, such as dyes or radioactive
tracers (R. Marzolf and C. Bowser, pers. comm.).
The use of the hollow jet valves (the release structure
for the ROW) created such a signature. The valves ejected
4 plumes of aerated water 10 m above the tailwater pool.
Combined with the draft rube discharges from the
penstocks, the higher discharge was more turbulent than
normal discharges. Turbidity and total suspended solids
increased from 0.2 to 0.6 NTU and 2 to 19 mg/L,
respectively, during the test flood (US Geological Survey
1996). The effects of spray and turhulence from the hollow
jet valves immediately oxygenated the tailwaters, resulting
in increased mean DO saturation from 79% to 105% (Fig.
7). Typically, rc, DO and pH reflect fluctuating diurnal
patterns that develop in the highly productive 25 km stretch
of normally clear, lower flows (Angradi et at. 1992, Ayers
and McKinney 1996). Respiration of Cladophora
glomerata and other primary producers and consumers
contribute to diel pH and DO fluctuations, while rc
responds to insolation. During the test flood, diurnal pH
patterns were attenuated, demonstrating the reduction of
respiration due to lower light availability (Fig. 7) resulting
from higher discharges, greater turbulence, deeper water,
and increased drift (M. Yard and D.L. Wegner, pers.
comm.). Diurnal pH and DO fluctuations recovered quickly
(within hours) once lower discharges recommenced,
although respiration was reduced from pre-flood levels.
Diurnal pH fluctuation levels had returned to pre-flood
levels by late April 1996. During the test flows, diurnal DO
patterns, though still present, were overshadowed by jet
valve aeration. Conductivity reflected short-term wind
induced seiche effects and higher salinity of the broader
withdrawal plumes in the forebay.

Conclusions and Management Implications
Given the context of antecedent conditions, these
data demonstrated significant impacts on reservoir and
downstream water quality. The most influential factors
were the magnitude and composition of the 20WI; followed
by the location, magnitude, timing, and duration of dam
discharges, though not necessarily in that order. Had the
test flood not occurred during the hypolimnetic upwelling,
or had the ROW not been used, the penstocks alone could
not have substantially flushed the hypolimnion. The ability
of the penstocks to mix and entrain the hypolimnion is
considerably less under normal discharge levels.
Hypolimnetic refreshment requires longer discharges, or
the opportunity to release meromictic water may be
foregone without high and bi~level discharges. In the
reservoir we observed significant shifts in salinity and DO
gradients nears the penstock and ROW elevations as far as
100 Ian uplake. Fresher, more highly oxygenated water was
drawn into the middle-depths of the forebay from the CM
epilimnion and 20WI uplake. These more dilute conditions
persisted through 1997. Although of short duration, the test
flood affected Lake Powell limnology in a fashion that
provides insight into the dramatic shifts in SC and DO
alluded to in the 1980's historical data set (Hueftle &

Vemieu 1998).
In the tailwaters, jet valve aeration, attenuation of
primary productivity, and the trace of seiches and
meromictic discharge were conclusive evidence of the test
flood, though short-lived. Longer-term aquatic impacts on
downstream resources are addressed by Shannon et al.,
Stevens et aI., and Valdez et aI" this issue.
These effects are important to in-lake water quality
and determination of down-river water quality. Currently,
large discharges are likely to occur only during periods of
high lake levels and high inflows, thus, future high releases
will probably occur during periods of declining meromixis.
Should in-lake hypoxia or meromixis approach levels of
concern. however. the test flood demonstrated a mechanism
for their downstream release. Hypoxia, not always
associated with meromixis, could be managed with well-
timed ROW releases. Dam operations could influence the
banking or release of ion concentrations, DO, TOC and
other components that were not examined here, such as
biological components. But uplake and downstream effects
must be considered prior to future actions. Dam releases
could be used to avert these problems before they reach
hazardous proportions. For example, precise releases at
peak upwelling in February or March would require less
discharge volume to reduce meromixis than at other times
of the year.
This study of large and multi~level discharges from
GCD has implications for future reservoir, discharge and
down-river management opportunities. The demonstration
of impacts from large and multi.level discharges from Glen
Canyon Dam has implications for future large floods and
other management options that are pending. Retrofitting the
ROW with turbines is currently under consideration, and
this provides a more tenable option to all stakeholders for
allowing winter discharge of the hypolimnion without loss
of power production. It could also offset loss of thermal
mass for another future management option-- selective
withdrawal.
Installation of a selective withdrawal system (SWS)
is an option outlined by the Final EIS (Stanford and Ward
1996). Its purpose, via epilimnetic withdrawal, is to wann
the Colorado River to encourage mainstem spawning of
endangered native fish. Such action could produce
unforeseen thermal, chemical and biological changes above
and below GCD. Use ofhypolimnetic discharge may offset
some of these impacts, and continued investigations could
lead to more informed decisions.
The demonstration of the test flood results as well as
the effects observed during the 1980's spillway discharges
alludes to impacts we could expect from the operation of a
SWS. Operational changes will have limnological impacts,
and informed decisions will require a sound limnological
foundation for management of water quality resources.
Current knowledge of the strength, destination and quality
of winter underflows and inflows, strength of meromixis,
antecedent conditions and long-range considerations will be
required for infonned management in the future.

ACKNOWLEDGMENTS
We would like to acknowledge D.L. Wegner and the
Glen Canyon Environmental Studies office for their hard
work in conducting the Spike Flood. It would not have
been possible without them. Thanks to Bill Vemieu for
assistance in field collection and design, data management
and boat operations. For assistance in field collections,