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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the Colorado River significantly aggraded the channel
and deposited sediments that probably caused larger
debris flows to reach the river.

The debris flow of December 1966 began with 11 slope
failures in the Hermit Shale and Supai Group and
traveled 13 miles to the Colorado River. Calculated
flow velocity ranged from 10-18 ft/s, and the discharge
ranged from 9,200- 14,000 cfs. Water content of the
debris flow ranged from 24-33 percent, and sand content
from 10-15 percent. One boulder transported by the
flow weighed an estimated 47 tons, and boulders with
diameters in excess of 5 ft were common. Upon reaching
the Colorado River, the debris flow created a new fan
surface that significantly constricted the Colorado
River (Kieffer 1985).

DISCUSSION AND CONCLUSIONS

Similarities among the three debris flows studied are
indicative of the cause and nature of debris flows in
Grand Canyon National Park. All three flows were
initiated as slope failures in the Hermit Shale and
Supai Group, especially the Esplanade Sandstone. All
debris flows transported a poorly sorted mixture of
clay- to boulder-size particles wi th water contents
that ranged from 23-33 percent by volume. The largest
boulders transported ranged from 9 tons in the
Lava-Chuar Creek drainage to 37 and 47 tons in the
Monument Creek and Crystal Creek drainages, respective-
ly. Two of the three debris flows significantly
constricted the Colorado River at the tributary mouths.
The frequency of debris flows reaching the Colorado
River is tentative; however, available data suggest
that one debris flow reaches the Colorado River every
20-50 years in these drainages. A compilation of
historical information on flow events from Grand Canyon
tributaries, however, indicates that debris flows
occur more frequently throughout the park.

The bedrock geology of Grand Canyon National Park pro-
vides an ideal situation for the initiation of debris
flows. The high reI ief combined with differential
strength properties of the rocks results in a high
potential for slope failures. The most common source
of mobilized sediments for debris flows are the Hermit
Shale and Esplanade Sandstone. Other sources include
the Permian Kaibab Limestone, Toroweap Formation, and
Coconino Sandstone; the Cambrian Muav Limestone and
underlying Bright Angel Shale; and Quaternary basalts
in the western Grand Canyon. Dispersive and swelling

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