WSP00018

• File Number: 8220.101.10
• Description: Colorado River-Water Projects-Glen Canyon Dam/Lake Powel-Glen Canyon Adaptive Management
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/2003
• Author: USGS-GCMRC
• Title: Variations in Sand Storage Measured at Monumented Cross Sections in the CO River Between Glen Canyon Dam and Lava Falls Rapid Northern Arizona 1992-99
• Water Supply Pro - Doc Type: Report/Study

Background

In the early 1980s, agencies charged with
management of the Colorado River in Grand Canyon.
white-water rafters. and anglers became concerned
that flow releases from Glen Canyon Dam were
eroding sandbars that are critical to the riparian
system in Grand Canyon National Park, Concern
about sandbars facused on potential degradation by
unsteady dam releases for power generation. In 1982.
the Bureau of Reclamation (BOR) began coordinating
a comprehensive program of investigaIions-Phase I of
GCES-IO determine the effects of dam releases on the
downstream riverine environment. During Phase I.
unexpected high flows prevented adequate study of
flows typical of the dam's powerplaRt operating
criteria, After a review of Phase 1 by the National
Academy of Science. the U,S, Department of the
Interior directed that further studies be done, Phase II
of the GCES was initiated iR ] 988 to gather additional
data on specific dam operational elements and included
flow-monitoring and model-development programs,
On July 27,1989. the Secretary of the Interior
directed the BOR to prepare an environmental impact
statement (EISl on the operation of Glen Canyon Dam.
To protect the downstrcam resources until completion
of the EIS, interim operating criteria and a monitoring
program were implemented in August] 991. The EIS
was completed in March 1995. and the Record of
Decision (ROD) on the opermion of Glen Canyon Dam
was signed on October 9. 1996, The ROD stipulated
new operating criteria for the dam based on the
preferred modified low fluctnating now (MLFF)
alternative presented in the EIS. Under the MLFF
alternative, limits were set on maximum and minimum
daily flow releases and on the rates of increase
(upramp) and decrease (downramp) of releases,
Specifically. the maximum now of 708 m'/s was
imposed in order to conserve sediment in Grand
Canyon. especially in the Marble Canyon area, The
downramp limit of 42 m'/s was imposed to reduce
beach erosion in the Marble Canyon reach that resulted
from rapid leaching of water in the beach sand.
Because it was assumed that limiting maximum
releases would cause portions of sandbars above the
normal peak stage to not be rebuilt; sediment to
accumulate at low elevations, including backwaters and
camping beaches; and return-current channels to
become filled with sediment and evemually overgrown
with vegetaIion. the MLFF also included habitat

maintenance flows (high. steady releases within
powerplant capacity) to re-fonn backwaters and
maintain sandbars (U,S, Department of the Interior.
1995), The EIS concluded thaI Glen Canyon Dam
reduced the sedimem-transport capacity of Ihe river to
a greater degree than it reduced the supply of sediment.
and thus, sediment would accumulme over time (U,S,
Department of the Interior, [995), Using MLFF
operating criteria. the EIS predicted that there would be
a 73-percent probability of sand accumulating in the
main channel over a 50-year period, Topping. Rubin.
and Vierra. 2000; Topping. Rubin. Nelson, and others.
2000; and Rubin and Topping. 200] have concluded
that multiyear sediment accumulation does not occur.
Beginning in 1992. as a parI of the GCES Phase II
interim-flow monitoring and model-development
programs. the USGS established a network of
monumemed cross sections between the confluence
of the Paria and Colorado Rivers and Lava Falls Rapid.
with greater emphasis on the reaches downstream
from the two largest tributaries-the Paria and Little
Colorado Rivers, Measurements were designed around
three hydrologic conditions in the year: (I) before the
snowmelt runoff in trihutaries in early spring. (2) after
the snowmelt runoff in late spring or early summer. and
(3) after summer rains in the fall, Between 1993 and
1996. additional cross sections were added between
Badger Creek Rapid and the connuence of the
Colorado and Little Colorado Rivers. and between
just upstream from Bright Angel Creek and Lava Falls
Rapid, In 1997 the number of cross sections was
reduced; sections in reaches that showed little to no
change were discontinued. and an index section was
chosen for each pool in the remaining reaches,
The measurement frequency of these index sites also
was reduced from three times per year to twice per
year: (I) in April to capture infonnation on deposition
of sediment from winter Iributary floods. and (2) in
September 10 capture infonnaIion on deposition of
sediment from late summer runoff in tributaries.
One purpose of the cross-section network was to
provide data on bed-elevation changes to compare to
computations of bed evolution made using multi-
dimensionalllow. sediment-Iransport. and bed-
evolution models, The network proved useful for this
purpose (Wiele and others. 1996). A second purpose of
the cross-section network was to provide a direct.
empirical method for monitoring the amount of sand
stored in Ihe channel to detennine if a multiyear
accumulation of sediment was occurring, Data indica-

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Introduction 3