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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DEBRIS FLOWS FROM TRIBUTARIES OF THE COLORADO RIVER
GRAND CANYON NATIONAL PARK, ARIZONA

Debris flows, slurries of clay- to boulder-size
particles, are a major process of sediment transport to
the Colorado River from ungaged tributaries in Grand
Canyon National Park, Arizona. Debris flows are
runoff events of large magnitude and short duration
that occur infrequently. They are the source of
potentially large volumes of sand for beaches on the
Colorado River. By forming debris fans at tributary
mouths, these flows create and modify hydraulic
controls (rapids) on the Colorado River.

By Robert H. Webb
Water Resources Division
U.S. Geological Survey
Tucson, Arizona

INTRODUCTION

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A potentially large source of sand for Colorado River
beaches in Grand Canyon National Park, Arizona, is
derived from sediment transported from small tributary
drainages (Figure 1). Little is known about the annual
sediment yield from these drainages, and existing
methodology for predicting sediment yields from small
basins is not designed for high-relief basins with a
large potential for slope failures. The key to
estimating sediment transport is an understanding of
the sediment transport process.

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A previous flood report (Cooley et ale 1977) and recent
mapping of alluvial deposits in tributary canyons
during this project indicate that debris flows are the
dominant process of sediment transport in small
drainages in Grand Canyon National Park. Debris flows
are common in arid and semiarid regions, but their
importance in supplying sediment to the Colorado River
has not been previously recognized. The purpose of
this report is to document the occurrence of debris
flows in Colorado River tributaries and to study three
tributary canyons in detail for debris-flow frequency
and the magnit~de of recent events.

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METHODS

Debris flows are flowing water-based slurries of poorly
sorted clay- to boulder-size particles (Costa 1984).
Debris flows typically have volumetric water content of

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