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

in areas of predominantly south-facing slopes. Irrespective of rock type,

very steep slopes typically are barren. Metamorphic rocks tend to weather

to a more clay-rich and mica-rich finer-grained regolith, especially on

gentler slopes. Predominantly north-facing and metamorphic rock slopes

commonly possess better developed pedogenic soils. Steeper rock slopes

have many common attributes regardless of rock type. The foliated meta-

morphic rocks produce larger rock fragments which tend to be flaggy or

slabby. The variable character of these surficial materials affects their

mass-wasting properties because of differences in shape and size of particles,

porosity and permeability, internal friction between particles, and vege-

tat ion cover.

Nature of Hazardous Geomorphic Processes

Generally speaking, two types of hazardous geomorphic processes occur

in the Big Thompson Canyon area. These include various mass-wasing pro-

2
cesses and fluvial transport as well as redeposition of surficial mate-

rials. Mass-wasting processes include soil creep, landslides, rockfalls,

rockslides, debris avalanches and debris slides (Varnes, 1958). Fluvial-

transport processes include sheet erosion of fine-grained, unconsolidated

surficial materials, erosional downcutting of drainage ways, fluvial mobili-

zation of larger debris on hills lopes (including vegetation and manmade

effects), and subsequent deposition or movement of debris on debris fans or

in major stream courses where it can be carried downstream. In either case,

the most common problems relating to human activities arise from impact of

moving earth materials and vegetation on structures, failure or erosion of

2
According to Colorado statute 106-7-103 (8) C.R.S., waterflooding is not
considered to be a geologic hazard. For a discussion of the State of
Colorado's involvement in geologic-hazard area identification and legal
definitions of geologic hazards, see Rogers and others, 1974.

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