7387

• UCREFRP Catalog Number: 7387
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Executive Summaries of Technical Reports.
• USFW Year: 1988.
• USFW - Doc Type: \
• Copyright Material: NO

and water discharges are estimated to be 3,100 and 900
cfs, respectively. The debris flow was composed of
30-35 percent sand and carried boulders that were about
1-2 ft in diameter. The largest boulder weighed an
estimated nine tons.

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Two debris flows occurred in the last 25 years in
Monument Creek drainage. A storm on July 27, 1984,
initiated an avalanche and subsequent debris flow that
reached the Colorado River. Some evidence indicates a
debris flow occurred in the early 1960s, and still
older debris-flow deposits were radiometrically dated
at about A.D 1780. Lack of correlation with downstream
deposits, however, precluded using this latter date for
determining frequency of events.

The debris flow of 1984 in Monument Creek drainage
began as an avalanche from the Esplanade Sandstone of
the Supai Group 2,000 ft above the channel. A
20-foot-high debris darn resulted and had not been
breached as of 1986. The debris flow traveled 2.8
miles to the Colorado River at a velocity of 11-13 ft/s
and had a peak discharge of about 3,800 cfs. The water
content of the flow ranged from 27-34 percent, and the
sand content from 30-40 percent. One boulder that was
transported during the flow weighed an estimated 37
tons.

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This flow created a new fan surface at the Colorado
River that significantly constricted Granite Rapid. We
estimated volume of the sediment transported onto the
fan and into the river on the basis of four
hypothesized scenarios of the fan geometry after
deposition of debris-flow sediments. The most likely
volume of sediment transported onto the fan and into
the river is 300,000 cubic feet. In 1986, the debris
fan was completely devoid of particles smaller than 16
mm in diameter, which suggested that all finer
particles (including sand) were transported quickly
into the Colorado River. Assuming an average sand
content of 35 percent, the estimated volume of sand
entering the river is 84,000 cubic feet, with a range
for all scenarios of 56,000-150,000 cubic feet.
Estimates of the volume of transported sediment and the
upstream discharge indicate that the fan was created in
1-3 min during the first pulse of the debris flow.

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The Crystal Creek drainage has averaged a minimum of
one debris flow reaching the Colorado River every 50
years. A large flow in December 1966 (Cooley et ale
1977) has been the only one to reach the Colorado River
in this century. Small debris flows that did not reach