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

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analysis of scarred trees, and historical photographs
provided a control on dating the ages of events.
Evidence for all events, however, has not necessarily
been preserved in the tributaries. We therefore
estimated the minimum frequency of debris flows.

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Simplified hydraulic formulas were used to calculate
flow velocities and discharges for debris flows
(Pierson 1985). We calculated velocities from runup
evidence of the velocity head, which is preserved in
sites where an obstacle is oriented perpendicular to
the flow direction. Superelevation evidence, which is
found where the flow surface is elevated on the outside
of bends due to centrifugal forces, was also used to
calculate velocities. The methods provide a conserva-
tive estimate of the actual velocity of debris flows
(Pierson 1985). Discharge was calculated as the
product of velocity and cross-sectional area. Project
personnel collected 5- to 10-pound samples of debris-
flow matrix in order to reconstitute the water content
of the debris flow using methods described in Cooley et
ale (1977) and Pierson (1985). Uncertainty in the
reconstituted water content by volume for each sample
was 1-2 percent. Particle-size distributions were
obtained by combining sieve data with point-count data
obtained in the field. The two methods yield
numerically equivalent particle-size distributions
(Kellerhals and Bray 1971).

DEBRIS FLOWS IN THREE TRIBUTARIES

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Evidence for at least five prehistoric and three
historic debris flows is preserved in the Lava-Chuar
Creek drainage. Of the historic debris flows, one
occurred between 1916 and 1966, another in December
1966, and the last between 1973 and 1984. Debris flows
have reached the Colorado River on an average of every
200 years during the last 1,500 years and every 20-30
years since 1916. Debris flows may reach the Colorado
River more frequently now because some prehistoric
debris flows may not have overtopped the terraces to
leave depositional evidence.

The debris flow of 1966 in the Lava-Chuar Creek
drainage began as slope failures in the Permian Hermit
Shale and Permian and Pennsylvanian Supai Group and
traveled 6.5 miles downstream to the Colorado River. It
had a velocity of 12 ftjs and a discharge of about
4,000 cubic feet per second (cfs) near the Colorado
River. The average water content of the flow was
estimated to be 22.5 percent: hence, the peak sediment

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