WSP12639

• File Number: 8220.101.10.A
• Description: Colorado River - Water Projects - Glen Canyon Dam-Lake Powell - Glen Canyon AMWG
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/2004
• Author: Schmidt - Topping - Grams - Hazel
• Title: System-Wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor Between Glen Canyon Dam and Bright Angel Creek - Arizona - Final Report - 10-01-04
• Water Supply Pro - Doc Type: Report/Study

001593

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prior 2 years. Degradation caused by the latter
flood was probably related to erosion of
boulders in Bright Angel Rapid that caused a
long-term shift in the stage-discharge relation.
There was a small amount of degradation
during the rising limb of the 1996 Controlled
Flood, because the stage-discharge relation
shifted by -0.35 m. Degradation at a slow rate
occurred between 1969 and 1971 and between
1974 and 1983, because Bright Angel Rapid
was presumably eroded to some degree.
Similar patterns of aggradation and
degradation were measured at the Lower
Marble Canyon gage between 1983 and 2000
(Fig. 24). Here, aggradation in the measure-
ment reach occurred during the few days when
floods from the Little Colorado River, located
approximately 1 km downstream (Fig. 15C),
entered the Colorado River. These tributary
floods temporarily increased the water-surface
elevation at the gage, decreased the velocity,
and induced bed aggradation. Ponding of the
flow was measured on January 22 and 23,
1993, during a flood from the Little Colorado
River, when the stage-discharge relation
shifted +0.64 m. Although not measured at the
time, a flood from the Little Colorado River
that occurred on January 12, 1993, probably
shifted the stage-discharge relation by approxi-

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Figure 24. Graph
showing mean bed
elevation and the
discharge of water
measured at the Lower
Marble Canyon gage
cableway between
1983 and 2001. Arrow
indicates January
1993 flood in Little
Colorado River,
located immediately
downstream from the
cableway.

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mately 1 m, because this flood exceeded the
January 23 peak by about 40%. These ponding
eve:nts caused bed aggradation of about 0.3 m
under the cableway, and the bed did not de-
grade to its pre-1993 elevation again until
August 1997.

5.3.3 Changes in Main Channel Pools Offshore
from Eddies between 1992 and 2000

Annual measurements of 16 main channel
pools between 1992 and 2000 by NAU further
confirm that there has been no system-wide
acc:umulation of fine sediment during the era
of ,environmental management. In fact, these
data indicate that there was probably a pro-
gressive loss of bed sediment. Many of these
pools scoured during the 1996 Controlled
Flood, remained scoured during the experi-
mental releases of 1997, and filled when dam
releases decreased in 1998 (Fig. 25). The
average trend in bed elevation between 1992
and 2000 is negative (p=9.8x10-4). This loss of
sediment included gravel as well as sand,
because pipe-dredge samples from these pools
inc:luded both.
The data from each of the 16 pools were
also normalized and averaged into one time
series, because each pool is only a few channel

5.0 Changes in the Topography of the Main-Channel Bed 35