WSP09131

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 2/21/2000
• Title: Experimental Flood Effects on the Limnology of Lake Powell Reservoir Southwestern USA
• Water Supply Pro - Doc Type: Report/Study

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Hue.ft1e and St.euens: Test Flood Effects on Lake Powell

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dam (Fig. 1). It provides 3 routes of release for
the reservoir's water. Eight penstocks located
70 m below full pool elevation are the primary
release structures. These can release a
maximum of 940 m3/ s to the 8 turbines for
power generation, but are constrained to
892 m3/s. The penstock draft tubes release
below the surface of the tailwater pool, limiting
aeration effects. Two alternate release
structures may be used for greater discharge
capacity, but both bypass power generation and
their use is avoided, The ROW are located 99 m
below full pool (29 m below penstock outlets)
and can discharge 424 -566 m3/s. Their greater
depth facilitates hypolimnetic discharge, and
they have been used on 7 occasions since 1963.
The spillways draw from the epilimnion near the
lake's surface at a depth of 16 m below full pool,
although the lake has been below the spillways'
operational levels for over half the lake's history.
The spillways have a capacity of 5890 m3/s to
accommodate a 100-year flood event, and have
only been used in 1980, 1983 and 1984 (USSR
1970, 1995).
Lake Powell is one of the largest U.S.
reservoirs; located in southern Utah and
northern Arizona, southwestern USA. (Fig. 2). It
fIrst reached full pool in 1980, and has a
maximum depth of 160 m, a surface area of
653 lan', a length of 300 lan, a volume of
32.1 lan3 and approximately 3200 lan of
shoreline at the full pool elevation of 1128 m
amsl (USSR 1970, 1995). The region has an arid
continental climate-- annual precipitation is
200 mm/year and pan evaporation is
1800 mm/year (Potter and Drake 1989),
Lake Powell is an oligotrophic lake (Potter
and Drake 1989) with low nutrient levels; mean
total phosphorus is 0.01-0.02 mg-P/L, total
Kjeldahl nitrogen = 0.16-0.2 mg-N/L. Results
from the 30+ year long-term Lake Powell
integrated water quality monitoring program
(IWQP) identify Powell as a warm meromictic
reservoir; it has never completely mixed since its
formation. It has a chemocline that persists
near the depth of the penstock withdrawals.
This meromictic hypolimnion, or
monimolimnion, contains relatively stagnant
water with elevated salinity (750 JlS/cm to
1200 JlS/cm), cold temperatures (6-90C) and
depressed DO (1.5-7 mg/L).
A previous period of meromixis at Lake
Powell was disrupted by high inflows and
multiple-level discharges in the 1980's during 5
years of exceptionally high inflows. The
spillways (near the surface) and the ROW were
operated on several occasions for extended
periods in 1980 and from 1983 to 1986.

Combined with 3 years of high flow-through and
multiple level withdrawals, the lake achieved a
unique level of homogeneity in June 1985, with
a conductance gradient 2.8 times less than the
average for the lake's history. Data collection in
the 1980's, however, was sporadic, with only 2
to 5 lake-wide collections per year. Trends were
discerned, but relationships between dam
operations and uplake processes were less clear.
It was expected that analyses of the test flood
results would clarify some of the effects
observed in the 1980's,
Data Collection and Sampling Design
Historical and ongoing data from the IWQP
were used, augmented with higher spatial and
temporal resolution data near the dam
surrounding the test flood (Fig. 2). The IWQP
includes 25 long-term monitoring stations, eight
that have been sampled since 1964. The test
flood was bracketed by 2 full-lake quarterly
IWQP sampling trips in the weeks of 1 March
and 6 June 1996. These included 25 stations in
the Colorado, Escalante and San Juan River
arms of Lake Powell. Using a Hydrolab Surveyor
H20 multi-parameter submarine sonde, profIles
of temperature (T"C), specific conductance (SC),
DO, pH, and turbidity were collected at depth
intervals of 0.5 to 5 m at each station. Water
chemistry samples were collected at 13 of these
stations and analyzed for nutrient and major
ion (APHA 1992) in the major stratigraphic
layers. Secchi disk readings and biological
samples of chlorophyll, phytoplankton, and
zooplankton were collected at the surface. The
IWQP also includes monthly sampling for all the
above parameters at the Wahweap forebay
station, and at the OCD and Lees Feny
tailwater stations.
The IWQP data was augmented with
6 additional physical profIles in the forebay
immediately before, during and after the test
flood on March 2200, 24th, 27th, and April 2nd,
3"', ~d 5th, 1996. Synoptic channel profiling
was conducted at 4 stations from the forebay
uplake to river lan 90 (Oak Canyon) on March
2200 and 27th, April 2nd and 5th, high winds,
however, truncated some of these efforts,
Chemical and biological samples were collected
at the forebay station (2.4 kIn uplake from the
dam) on 22 March and 5 April. An additional
lake-wide collection of physical profIle data was
taken at 17 stations on the Colorado River arm
of the reservoir to its inflow the week of 20 April
1996.
Higher resolution temporal data for the
flood included 3 permanently deployed Hydrolab
Recorders within and below the dam and at Lees

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