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

22 The State of the Colorado River Ecosystem in Grand Canyon

ies, this program extended from July through December
1983. On the mainstem, this program included the
periods from July through December 1983 and October
1985 through January 1986. All suspended-sediment
samples collected under this program were analyzed for
grain size to allow use in constructing sand budgets.
The sand budget for the Colorado River in Marble
and Grand Canyons used in the EIS was constructed by
Randle and Pemberton (1987) and Pemberton (1987).
For tributalY sand input, they constructed stable sand-
rating curves by using all of the historical and 1983
data from the Paria River, the Little Colorado River,
and Kanab Creek. They also included an estimate for
the sand supply from the lesser tributaries. Pemberton
(1987) developed stable sand-transport rating curves at
the five mains tern locations based on the USGS 1983-86
data, and the EIS states, "The sand transport equations
of Randle and Pemberton (1987) and Pemberton (1987)
were used for these computations" (U.S. Department of
the Interior, 1995, p. 95) in reference to the sediment
budget presented in figure III-IS of the EIS (and repro-
duced here as fig. 2). Therefore, the EIS sediment bud-
get was based on the assumption of stable sand-transport
rating curves. Results of recent studies presented in the
following section suggest that this assumption is incorrect
for the Colorado River below Glen Canyon Dam.

30

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-10

1964

1968

1972

1976

1980

1984

1988

Figure 2. Reproduction of figure 111-15 from the final
environmental impact statement (EIS) (U.S. Department of the
Interior, 1995), which shows the sand budget as computed
by Randle and Pemberton (1987). Recent studies refute the
conclusion of the EIS that sand accumulates on the bed of the
Colorado River over multiple years under normal dam operations.
(Phantom Ranch is the location of the Grand Canyon gage.)

Studies Since 1996 That Refute
the Environmental Impact
Statement Findings

1992

Research and monitoring conducted during and
after the 1996 BHBF experiment, also known as the
1996 controlled flood, have led to several findings that
refute the EIS predictions for sand conservation and
suggest that the implementation of this strategy has
not led to sustainable restoration and maintenance of
sandbars in either .Marble or Grand Canyon. Instead,
the canyons' sandbars continue to erode (figs. 3-6). The
primary results of several of these studies are briefly
summarized below:

· Rubin and others (1998) and Topping and oth-
ers (1999) showed that the sand supply during
the 1996 BHBF was not as great as was assumed
before the experiment and that the sand on the
bed of the river and in suspension coarsened
dramatically as the upstream supply of sand
decreased over time during this flood. This pro-
cess led to flood deposits that coarsened vertically
upward (i.e., inversely graded deposits).

· Topping and others (2000a) demonstrated that
the grain size of sand on the bed of the Colorado
River can change by over a factor of four as func-
tions of tributary resupply of finer sand and higher
dam releases that winnow the bed and that this
factor-of-four change in bed-sand grain size cor-
responds to a change of two orders of magnitude
in the concentration of sand in suspension (for the
same discharge of water). Identification of this
dynamic process precludes the use of stable sand-
transport relationships in the Colorado River,
thus invalidating the approach used to construct
the sand budget in the EIS. Topping and others
(2000a) also showed that Randle and Pemberton
(1987) incorrectly predicted sand accumulation
in the Colorado River because the data they used
to verify their modeled stable sand-export rela-
tionships were from periods in the mid-1980s,
when sand in the river was anomalously coarse
and sand-transport rates were anomalously low
following prolonged releases above powerplant
capacity between 1983 and 1986.

. Rubin and Topping (200 I) showed that sand
transport in the postdam Colorado River in
Grand Canyon is regulated by both the discharge
of water and the grain size of the sand available
for transport in suspension. This information also