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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Fluvial mobilization of larger debris on hillslopes. In addition to

the progressive sequence of fluvial transport phenomena discussed above,

some larger debris can be moved by water running off moderate to steep

hillslopes. The most common mode of this kind of debris movement involves

the removal of physical support for large rock fragments or vegetation by

local erosion of the finer grained materials on which both rest. This can

result in rock movements similar to rockfalls and rockslides. Fallen trees,

small woody debris and duff can be carried or dislodged by rapidly moving

water. Although vegetation-debris movement may seem unimportant, it can

dam small stream channels, divert sheet runoff, and locally concentrate

water movement.

Debris fans. The culmination of the above discussion of fluvial trans-

port phenomena is concerned with their process relationships to deposition

and movement of material on debris fans. A debris fan. is a triangular-shaped

landform created by deposition of fluvially transported material at the con-

fluence of a tributary stream with a higher order stream. In the Big Thompson

Canyon, debris fans or vestiges of them are found at nearly every stream con-

fluence including most of the small ephemeral drainages. The processes that

form or modify debris fans are summarized in the simplified process models

shown in Figure 1. In the first case (A), which typifies most debris fans in

the Big Thompson Canyon prior to the July 31 - August 1 rainstorm, the tribu-

tary stream has incised the debris fan in a well-defined stream channel. In

most places, the bedload of this tributary stream is composed principally of

boulders; the finer grained bedload constituents have been carried away

presumably during periods of normal seasonal high runoff. As indicated on the

diagram, debris fans commonly make attractive development areas by virtue of

their modest slopes, relative ease of excavation, and potential for stream-

side building sites away from the mainstream flood plain. In the second

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