FLOOD02366

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Big Thompson Flood of 1976 Field Trip Guidebook
• Date: 1/1/1976
• Prepared For: Annual meeting of Geological Society
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

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GEOMORPHOLOGY OF GEOLOGIC HAZARDS IN THE BIG THOMPSON
CANYON AREA, LARIMER COUNTY, COLORADO

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James M. Soule
Colorado Geological Survey
1313 Sherman Street
Denver, Colorado 80203

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Introduction

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Slope inclination, aspect, rock type and local meteorology appear to

be the dominant influences on modern geomorphic processes in the Big

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Thompson Canyon area and in other similarly situated canyons of the Colo-

rado Front Range. This preliminary report describes several physical

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attributes of this mountain landscape as they relate to actual and potential

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geologic hazards. Current studies by the Colorado Geological Survey and the

U. S. Geological Survey to investigate hazardous geomorphic processes in the

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Big Thompson Canyon and delineate geologic hazard areas are underway. This

work should assist in planning the safe restoration of the Big Thompson

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Canyon area in the aftermath of recent flooding on July 31 - August 1, 1976.

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This work is also intended to serve as a model planning guide for other

Front Range canyons.

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The Big Thompson River and its tributaries are deeply incised into Pre-

cambrian crystalline and metamorphic rocks. Composition of this terrain

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varies from tonalite and granodiorite to biotite-staurolite-grade metamorphic

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rocks and amphibolite (Braddock and others, 1970). Weathering and disinte-
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gration of bedrock usually form a thin grus mantle on gentle to moderate

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granitic slopes. Steeper slopes on these crystalline rocks commonly are

strewn with boulder rubble and are nearly barren of vegetation, especially

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As used in this paper, slope modifiers are defined as follows:
Very gentle slope: less than one percent (slope)
Very gentle to gentle slope: 1 to 5 percent
Gentle to moderate slope: 5 to 15 percent
Moderate to steep slope: 15 to 30 percent
Steep slope: greater than 30 percent
Very steep slope: much greater than 30 percent

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