Mount Adams, Washington:
October 20, 1997 Debris Avalanche -
First Observations

INFORMATION STATEMENT
October 23, 1997

A large rock avalanche occurred Monday, October 20, 1997, on the east side of Mount Adams, Washington. Based on seismic signals, the avalanche began at 12:31 AM Pacific Daylight Time and lasted about six minutes. There were no seismic precursors.

On October 21, a USGS scientist inspected the avalanche deposit from a small airplane. The avalanche originated at about 11,200 ft. elevation on the south face of The Castle, a prominent topographic knob at the head of Battlement Ridge. The source area forms an obvious, near-vertical scar roughly triangular in shape with sides about 300 meters in length. The summit of The Castle remains intact. The avalanche descended the Klickitat Glacier icefall and left a thin veneer of rock debris on the steep upper part of the glacier. Below roughly 8,000-ft. elevation the deposit thickened. The avalanche traveled beyond the end of the Klickitat Glacier and continued roughly 2 kilometers down the valley of Big Muddy Creek, a tributary of the Klickitat River. The length of the avalanche track totals about 5 kilometers, and the width may exceed 1 kilometer in places. The average width is about 1/2 kilometer. Maximum deposit thicknesses may exceed 20 meters. The volume of the avalanche debris is probably between 1 and 5 million cubic meters.

The avalanche deposit temporarily blocked the flow of Big Muddy Creek, resulting in the formation of a small lake on avalanche debris [Image,270K,GIF]. By noon on October 21 the avalanche dam had breached, and flow in Big Muddy Creek did not appear unusual. Continuing hazards exist due to the threat of additional rockfall and additional damming and downstream flooding. However, these hazards exist primarily in unpopulated areas deep within the backcountry of Yakima Nation lands. No evidence suggests that hazards in populated areas far downstream have increased significantly.

This avalanche appears unrelated, except in the broadest fashion, to a similar-sized avalanche that occurred on the western flank of Mount Adams about seven weeks earlier (August 31, 1997). Both avalanches originated in areas composed of rocks evidently weakened by intense hydrothermal alteration. Both avalanches may have been triggered, in part, by wet subsurface conditions associated with late-season thawing of exceptionally heavy snowpack in conjunction with early-season storms. Neither avalanche was triggered by regional earthquake or volcanic activity.

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