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

and others. 1978; Kieffer. 1985), The downstream ends
of these pools are controlled by bedrock outcrops or
alternating debris bars that are the outwash from the
upstream debris fans (Howard and Dolan, 1981; Webb
and others. 1989; Melis and Webb. 1993; Melis and
others, 1994), Because the debris fans raise the bed
elevation (Howard and Dolan. 1981), large pools also
form upstream from rapids. The length of the pools is
determined by the spacings between debris fans. which
are controlled by bedrock geology (Howard and Dolan.
1981), These pools are recirculation zones and may
contain one or more eddies and low-velocitv areas. thus
providing an environment for sediment deposition
(Schmidt and Grar. 1990),
Prior to the closure of Glen Canyon Dam in 1963.
the Colorado River in Grand Canyon had highly
variable discharge and suspended-sediment
concentration annually, Between May and July. melting
of the snow pack in the headwaters produced mean
annual peak discharge of 2.420 m'/s, Except for brief
but occasionally substantial summer tributary !lash
!loods. !low in the Colorado River typically fell to less
than 200 m'/s after the snowmelt !lood subsided, Post-
dam !low regulation has removed seasonal-!low
variations by reducing Ihe frequency of discharges that
are greater than poweIplant capacity (940 mJ/s) and
increasing the overall range and rate of change of
discharge during each day.
The average sediment load for the Colorado River
at Lees Ferry before Ihe dam was 60 million metric
tons per year (Webb, Wegner, and others. 1999), The
suspended-sediment load of the post-dam Colorado
River has been reduced by 95 percent (Webb. Wegner.
and others, 19991 and is primarily supplied by the
major tributaries. the Paria and Lillie Colorado Rivers.
and numerous smaller, un gaged tributaries. The mean
annual supply offine sediment was 2,74 million meIric
tons for 1947-76 from Ihe Pari a River and 8.4 million
metric tons for 1947-72 from the Lillie Colorado River
(Webb. Wegner. and others. 1999), The smaller
tributaries create and maintain the debris fans and
rapids on the Colorado River and serve as important
sources of fine-grained sediment (Powell. 1895;
Hamblin and Rigby. 1968; Dolan and others. 1974:
Howard and Dolan. 1981: Keiffer, 1985). EstimaIing
the sediment yield from Ihese smaller tributaries is
complicated because sand is contributed by a
combination of !luvial and hillslope processes (Webb
and others, 2000). Using a sediment-yield model for
debris flow. Webb and others (2000) eSIimaIe that

approximately 2,0 million metric tons of total sediment
enters the river from the ungaged tributaries between
Glen Canyon Dam and Diamond Creek (river mile -15
to 226),

Controlled Releases and Gaged Tributary Inputs

Changes in area measured at cross sections within
the study area are affected by controlled flows from
Glen Canyon Dam and sediment input from tributaries,
Under the MLFF preferred alternative specified in the
EIS. peak releases are limited to 708 m3/s. minimum
releases are limited to 227 m'/s during the day and
142 m'/s at nighI. and the maximum daily fluctuation
or range in flow is limiIed to 227 m'/s, The preferred
alternative also provides for short duration. high-flow
releases to accomplish specific management objectives
and to simulate some of the dynamics of a natural river
(U,S, Department of the Interior. 1995). In March-
April 1996. a controlled high-flow release experiment
(1996 controlled flood) was conducted, A flow of
1.275 m'/s was released from Glen Canyon Dam for
a period of 7 days to determine the effectiveness with
which discharges greatly in excess of the normal
poweIplant-restricIed maximum could renew the
riparian environment along the Colorado River. Several
papers on the controlled flood are compi led in Webb.
SchmidI. and others (1999),
For this study period. the box plots shown in
tigure 2 graphically summarize the distribution of
maximum. minimum. and range (daily maximum -
daily minimum) of daily discharge values for water
years 1992-99 at the USGS streamflow-gaging station
Colorado River at Lees Ferry (09380000; tig, I I.
approximately 16 miles downstream from Glen
Canyon Dam, The distribution of maximum and
minimum daily discharge values is lower during water
years 1992-95 Ihan during water years 1996-99,
Beginning in waIer year 1996, there is an increase in
the annual median daily maximum and minimum
discharge. The annual median daily discharge
decreased in water year 1999. but was still larger than
medians of the earlier period (1992-95), The maximum
daily discharge at this staIion occurred during the 1996
controlled flood. and the minimum daily discharge
occurred in water year 1992, Table I shows annual
summary statistics for discharge aI Lees Ferry,
Suspended-sediment discharge from the Glen Canyon
reach, measured at the Lees Ferry gaging sIation, is
negltglble and Iherefore is not included in table I or
other analyses in this report,

~&io1tJ

Introduction 5