Mount Adams, Washington:
August 31, 1997 Debris Avalanche -
First Observations

Between about 10:00 AM and 2 PM (PDT), August 30, 1997 -- (Web note: date revised - see revised observations) -- , a large avalanche of snow, ice, and rock debris descended the southwest flank of Mount Adams, Washington. No known direct observations of the avalanche exist. This synopsis of the event is preliminary and is based on one hour of aerial reconnaissance on September 8 and one person-day of on-the-ground field reconnaissance September 11, supplemented by reports from local residents.

The avalanche headed at about 12,000 feet elevation in the cirque of the Avalanche Glacier on Mount Adams' southwest flank. It descended to about 6,300 feet elevation and traveled a maximum horizontal distance of about 3 miles. Most debris ponded behind glacial moraines at about 7400 feet elevation, but perhaps one tenth of the debris overtopped moraines and funneled down the valleys of Salt Creek and Cascade Creek. No part of the avalanche reached the Round-the-Mountain hiking trail. However, a small debris flow discharged from the toe of the avalanche in one location, traveled down the east fork of Cascade Creek to about 6000 feet elevation, and caused minor damage to the Round-the-Mountain trail. Streamflow observed below the avalanche on September 11 did not appear abnormal.

Total volume of the avalanche is probably between 1 million and 10 million cubic meters. A more precise volume estimate will require more detailed work than has been possible to date. Most of the avalanche volume appears to consist of snow and glacial ice, although most of the avalanche deposit is veneered by muddy rock debris. The deposit surface is quite unstable -- much like a debris-covered glacier terminus -- and shifting of debris and ice is common.

A continuing hazard exists in the immediate vicinity of the avalanche deposit in the area bounded by Crofton Ridge to the southeast and Stagman Ridge to the northwest. In this area minor shifting of avalanche debris can be expected to occur, and sporadic rockfall will likely continue from the avalanche source area and deposit. Hazards are greatest along the channels of Salt Creek and Cascade Creek at elevations above 6000 feet and are less at lower elevations and away from stream channels.

No obvious trigger, such as an earthquake or volcanic activity, precipitated the avalanche. Instead, the avalanche is likely the result of long-term weakening of volcanic rocks and perhaps of a smaller precursory avalanche that involved only snow and ice. The rock exposed in the cirque of the Avalanche Glacier, where the avalanche originated, appears intensively altered by hydrothermal activity. Rock debris exposed on the avalanche deposit is dominated by fine-grained, clay-rich material. Few fragments of solid rock larger than 1 meter exist in the deposit.

Stratigraphic and cross-cutting relationships between deposited snow, ice, and rock debris suggest that the first pulse of the avalanche may have consisted mostly of snow and that a subsequent, larger pulse of glacial ice followed by broken, clay-rich rock arrived later. More work is needed to test this hypothesis.

The avalanche followed nearly the same path as a larger rock avalanche that occurred in 1921 and of still larger prehistoric avalanches. Recurring avalanches in this area result from the large volume of weak, altered rock in the areas occupied by the Avalanche and White Salmon Glaciers. Refer to USGS Open-File Report 95-492, "Volcano Hazards in the Mount Adams Region, Washington" for more information on past avalanche and volcanic activity at Mount Adams.

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