9628

• UCREFRP Catalog Number: 9628
• Author: Gloss, S. P., J. E. Lovich and T. S. Melis.
• Title: The State of the Colorado River Ecosystem in Grand Canyon - A Report of the Grand Canyon Monitoring and Research Center 1991-2004.
• USFW Year: 2005.
• USFW - Doc Type: \
• Copyright Material: NO

Influence of Glen Canyon Dam Operations on Downstream Sand Resources 27

the Interior, 2002) defined a sequence of events related
to sand inputs and retention that would trigger a 2-d,
42,000-45,000-ds experimental high flow in the follow-
ingJanuary (fig. 9). Significant sand inputs to Marble
Canyon that exceeded the triggering threshold for an
experimental high flow occurred during September-
November 2004. Instead of constraining operations
through December (a winter, peak-demand month) in
order to retain sand in Marble Canyon as laid out in the
2002 EA, a supplemental EA was prepared that allowed
for a hybrid of the first and second approaches to be
tested and evaluated. Approval of the supplemental EA
paved the way for the experimental high flow that began
on Sunday, November 21, 2004, when the Bureau of
Reclamation opened the bypass tubes of Glen Canyon
Dam for 90 h. The peak high flows ran for 2.5 d (GO h)
at about 41,000 cfs. Scientists will evaluate data col-
lected during and after the high-flow event to determine
whether or not this strategy succeeded in enlarging exist-
ing beaches and :;andbars.
Other dam operation scenarios may be more effec-
tive at retaining tributary inputs, such as Record of
Decision operations modified such that equal volumes
of water are released from the dam each month. Alter-
natively, a scenario of seasonally adjusted steady flows,
which was an alt,~rnative in the EIS process, may be
effective. Became of the severely reduced sand sup-
ply, howevel~ even during periods of minimum release
requirements of 8.23 million acre-feet (10,148 million
m3) per year the possibility exists that no operational
scenario will result in management objectives being
achieved for restoring sandbars, simply because of the
volume of water that must be released on an annual
basis. If so, other, more effective alternatives for restor-
ing and maintaining sandbars and related habitats may
need to be evaluated.
Sediment augmentation, one possible alterna-
tive, was eliminated during the development of the
EIS, partly because of the belief that sandbars could
be restored and maintained by constraining the hourly
ramping rates and range of daily dam operations and
partly because of concerns about contamination of sedi-
ment upstream in Lake Powell (Graf, 1985). Addition of
sediment-continuously, seasonally, or perhaps only dur-
ing floods-may ofier greater powerplant operating flexi-
bility and theref()re may cost less than further restrictions
on annual dam operations. To this end, the feasibility
of mechanically transporting fine sediment around Glen
Canyon Dam and introducing it into the Colorado River
below the dam i:; currently being investigated.

No

July 1-0 ct 31 ...............
Fine sediment inputs> 550,000 tons? .....,.......

Yes

Switch releases (after Sept 1) to
alternating, 2.week.long 8,000.cfs
steady and 6,500-9,000.cfs
fluctuating flows.

On Oct 31, inputs
> 1,100,000 tons?

Yes

Continue alternating releases.
Decide on Dec. 1 which conserves
more fine sediment and
continue this release.

On Jan. 1, > 880,000 tons
of fine sediment retained?

Yes

Initiate 2.d, 42,000-45,000-cfs
flow experiment in the first
week of January.

No

~

No

~

Figure 9. Sequence of events established in the autumn sediment
input scenario in an environmental assessment by U.S. Department
of the Interior (2002) related to fine-sediment inputs and retention
to trigger a 2.d, 42,000-45,000-cfs experimental high flow in
January. If fine-sediment inputs do not reach specified levels, then
modified low fluctuating flow (MLFF) operations, as specified in the
Record of Decision (ROD) (U.S. Department of the Interior, 19961.
are continued.