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

3

conductance (SC), DO, pH, and turbidity at depth intervals
orO.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 1994) in the major stratigraphic
layers. The LPMP also includes monthly sampling for all
the above parameters at the Wahweap forebay station, and
at the GeD and Lees Ferry tailwater stations.
We augmented the LPMP data with 6 additional
physical profiles in the forebay immediately before, during
and after the test flood, on March 22nd, 24th, 27th, and April
2", 3", and 5'", 1996. We conducted synoptic channel
profiling at 4 stations from the forebay uptake to river krn
90 (Oak Canyon) on March 22" and 27'", April 2" and 5'",
high winds, however, truncated some of these efforts.
Chemical and biological samples were collected at the
forebay station (2.4 Ian 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
ann 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 TM
within and below the dam and at Lees Ferry, 25 Ian below
the dam. These measured PC, SC, DO, and pH at half-
hour intervals. However, the high flows of the test flood
rendered some of this information unusable. These
Hydrolab ™ profiles provided the finest resolution and the
most consistent data sets--particularly at the greater depths
affected by the penstock and ROW withdrawals.
All Hydrolab instruments was calibrated using
standard solutions and established protocol (Hydro lab
1994) before and after each sampling period. For every 10
chemical samples we collected blanks, duplicates, and
spiked samples.
Analyses
We compiled, reviewed and analyzed the data using
SAS software (SAS Institute Inc. (996). Data were plotted

using SAS and Grapher@(Golden Software 1994), and
three dimensional (isopleth) graphics were generated using

Surfer@ (Golden Software, V. 6.04, 1996). Synoptic
channel profile isopleths plotted various parameters for
depth (in elevation) against river channel distance uplake
from Glen Canyon Dam to illustrate hydrodynamic
processes. Temporal isopleths plotted various parameters
using depth against time; and these facilitated long-term
trend analysis. An animation sequence of the lake-wide
conductivity isopleths since 1965 is available at
www.usbr.gov/gces.This demonstrates hydrodynamics,
underflows and discharges of the reservoir including
profiles of the test flood.

RESULTS AND DISCUSSION

Discharge Hydrograph and Lake Elevation
Prior to the test flood, the darn had discharged at
above average levels since June 1995 as a result of large
inflows that spring. Flows were increased from 280.
340 m3/s to 480-537 m3/s in June and maintained there until
October 1995, and thereafter averaged 340.425 mlls until
the test flood in 1996.
On 26 March 1996, penstock and ROW releases were
increased to 850 mlls and 425 m3/s, respectively (Fig. 3). A
total volume of 0.891 kIn3 was discharged during the test
flood; 0.626 Ian' from the penstocks and 0.267 Ian' from
the ROW. Following the experiment, discharges from the

darn were increased to high fluctuating levels of 450-
566 ml/s for the duration of the spring to accommodate the
large 1996 snowpack. Although we refer to the test flood as
the 7 days of high releases, the experiment included 8 days
of low steady flows surrounding the flood (patten e/ al.,
this issue) which also produced effects to lake and
tailwaters.
The test flood directly affected lake elevation. Over
the course of the experiment, between March 220d and
April 8th, reservoir elevation had a net drop of 0.98 m.
Although the reservoir dropped 1.12 m during the test
flood, the 4 days of 227 mlls discharges preceding and
following the 1,274 m3/s flood increased reservoir stage by
0.15 m. The lake elevation changes were slightly more
than anticipated because of a later onset of the high spring
inflows. Soon after the experiment concluded, the reservoir
elevation increased substantially. The sudden drop in lake
elevation required that water stored in the more eutrophic
side bays enter the mainstem (Thorton et al. 1990). Our
data suggests mainstem nutrient levels may have increased
throughout the reservoir in June 1996, accompanied by
increased chlorophyll.a and -c and pheophytin-a Trends
are not conclusive, but suggest further investigation.

Lake stratification and hydrodynamics: Antecedent
conditions
The previous decade's climate and inflow patterns
affected the limnological conditions prior to the test flood,
and understanding these is critical to interpreting the results
of the test flood on reservoir stratification and
hydrodynamics. From 1987 to 1994, Lake Powell's
drainage basin experienced extended drought; 6 of those
years were among Lake Powell's lowest inflows in the
reservoir's 33-year history. This resulted in a pronounced
monimolimnion with a pycnocline (density gradient)
resistant to mixing. This stratification was weakened by 2
high inflows (5'" and 6'" highest in the lake's history) in
1993 and 1995. These inflows introduced a large pool of
lower SC water for winter convective mixing in the
epilimnion.
Normal winter hypolimnetic processes are dominated
by partitioned underflows that form in the inflows and
migrate advectively toward the dam (Hueftle and Vernieu
1998). The first winter underflow (IOWU) forms in the fall
as a relatively warm, saline mass of dense water flows
along the former riverbed toward the dam, dispersing
through and thickening the monirnolimnion. The secondary
winter underflow (20WD) forms in the inflow at the peak
of winter, a cold, convectively mixed mass of relatively
cold, oxygenated and lower salinity water which follows
the IOWU down-lake. Although its density is rarely
sufficient to completely displace the hypolimnion, the
20WU may refresh the stagnant hypolimnion if it is of
sufficient magnitude and density, and dam discharges are
favorable. Most commonly, this 20WU reaches the
chemocline midway down the thalweg in the reservoir and
becomes an interflow (20WI), overriding or passing
through the hypolimnion, depending on its relative density.
It is then drawn into the penstock withdrawal zone. (Merritt
and Johnson 1978, Johnson and Merritt 1979, Gloss et al.
1980, Gloss and Reynolds 1981, Edinger et al. 1983,
Stanford and Ward 1986 and 1991). This 20W1 occurs
regularly, and its freshening potential increases with the
depth the density current achieves before diversion over the
hypolimnion. Preceding the test flood, the 20W underflow