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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peak discharges for Drake and also for the mouth of the canyon are esti-

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mated at 31,200 c.f.s., indicating the storm's location above Drake.

Approximate timing of flood peaks has been determined, in part,

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from eyewitness accounts. The Big Thompson peaked at about 8 pm at Glen

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Comfort, 9 pm at Drake, 9:30 pm at the Loveland power plant (about half-

way between Cedar Cove and Midway) and at 11 pm at the mouth of the canyon.

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The flood at Dry Gulch, near the head of the canyon, was in response to a

late burst of rainfall, for its peak discharge was recorded at 10:30 pm.

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The flood on the North Fork was due to heavy rainfall near Glen Haven.

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Its peak discharge near Drake came at 9:40 pm.

Velocities associated with these high flows were also extreme. Flow

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rates of 25 feet per second clearly were not uncommon. Stream banks and

much of U. S. Highway 34 were undercut and removed, trees and numerous

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structures including the Loveland power plant were swept away. The

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largest clast that can be documented to have moved has a maximum inter-

cept of 22 feet. The largest structure to be removed is the nine foot

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diameter pipe which spanned the mouth of the canyon.

Tremendous quantities of material were transported by the flood. It

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has been estimated by one worker that 40% of the flow volume consisted of
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debris. Most of the coarse debris was deposited in the canyon while most

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of the flotsam was carried completely out of the canyon._~

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In summary, it can be stated that an unusual set of atmospheric con-

ditions triggered a short-lived flood of catastrophic proportions in the

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Big Thompson Canyon. At the time of this writing the maximum discharge

is estimated at 31,200 c.f.s. Flood depths are thought to have exceeded

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20 feet at some locales and velocities greater than 25 feet per second

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are a certainty. Flow velocities on the steeper tributaries may have been

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