Mount Adams, Washington:
October 20, 1997 Debris Avalanche -
Updated Information

MEMORANDUM TO: The Record FROM: Dick Iverson, October 30, 1997 SUBJECT: Mount Adams Avalanche of Oct. 20, 1997, updated information statement

A large debris avalanche composed of rock and ice 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, USGS and U.S. Forest Service scientists inspected the avalanche path 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 avalanche source area forms an obvious, near-vertical scar roughly triangular in shape, with sides about 300 meters (1000 feet) in length. The summit of The Castle remains intact. The avalanche descended the Klickitat Glacier icefall, scoured ice along the way, and left a discontinuous veneer of rock debris in its path. Below roughly 8,000 feet elevation, rock deposition became more continuous. The avalanche continued about 2 km beyond the terminus of the Klickitat Glacier, buried the upper reaches of Big Muddy Creek, a tributary of the Klickitat River, and stopped at about 5600 feet elevation. The length of the avalanche path totals about 5 kilometers (3 miles). The path width averages about 1/2 kilometer and reaches 1 kilometer (0.6 miles) in places.

The avalanche deposit temporarily blocked the flow of Big Muddy Creek, resulting in the formation of a pond on the distal part of the avalanche debris. By noon on October 21 the avalanche dam had breached, and flow in Big Muddy Creek appeared muddy but not unusually high.

On October 24 at 7:29 AM Pacific daylight time, another block of rock fell from the oversteepend slope at the head of the October 20 avalanche scar. The seismic signal produced by this rockfall indicated that it was substantially smaller than the October 20 avalanche, and field observations subsequently confirmed this interpretation.

On October 25 scientists from the USGS and Yakima Nation made an on-the-ground field reconnaissance of the avalanche path and deposit. Some noteworthy findings of this reconnaissance are summarized as follows: (1) The distal part of the avalanche is covered by a veneer of rock debris but at depths greater than about 10 cm (4 inches) consists chiefly of a matrix of crushed, annealed glacier ice, impregnated with rock fragments mostly gravel-sized and smaller and including some clay. A lumped, 2-kilogram sample of this "ice conglomerate" matrix, excavated from three locations and then homogenized, consisted of about 40% rock and 60% ice by weight. These percentages yield a volumetric percentage of rock probably greater than 10% and certainly less than 20%, depending on the porosity of the crushed and annealed glacier ice. (2) The thickness of the distal part of the avalanche deposit, which appears to be the thickest part, probably averages about 10 m but may exceed 20 m ( 70 feet) in places. (3) The largest single rock clast visible at the surface of the deposit has a maximum length exceeding 20 m, volume of about 4000 cubic meters (5000 cubic yards), and an estimated mass of about 10,000 tons. This clast, like most other large rock clasts in the deposit, consists of both massive lava and lava-flow breccia, covered by a patchy surficial rind of snow and ice. (4) Rounded ice "boulders" up to several meters in diameter are also common in the deposit but are less common than rock clasts of comparable or larger size. Most ice boulders have adhering coatings of rock debris, and fragmentation and subsequent annealing of debris-coated ice boulders may be responsible for the continuous veneer of rock debris on the surface of the avalanche deposit. (5) Most rocks visible in the deposit have a monotonous surface color that is reddish dark brown. However, numerous other clasts have stains of bright red, orange, yellow, or white, presumably indicative of hydrothermal alteration or other secondary mineralization. The deposit also contains clasts that appear hydrothermally altered throughout. (6) Many clasts with brecciated textures have voids filled with ice, and a few altered breccias have voids filled with delicate gypsum crystals. (7) Prominent striations, similar to glacial striations and alligned in the direction of avalanche motion, are evident in some rocks embedded in ice conglomerate that was emplaced early in the depositonal process and subsequently overridden by additional avalanche debris. This evidence of sequential emplacement of deposits demonstrates that deposition did not occur en masse. (8) Multiple sets of prominent lateral levees are composed mostly of ice conglomerate and provide additional evidence for a sequence of depositional events rather than uniform avalanche cessation. (9) No evidence exists for any part of the avalanche having mobilized into a debris flow that continued downstream. (10) The volume of the avalanche debris is difficult to estimate accurately without more detailed field work and photogrammetric mapping. However, a rough estimate can be based on the observation that the part of the avalanche path with significant deposition covers perhaps 2 square kilometers (500 acres), and the average deposit thickness along this path is almost certainly less than 10 meters but probably more than 1 meter. A conservative mean thickness estimate of 2 m (7 ft.) yields a total deposit volume of 4 million cubic meters (5 million cubic yards). This is best regarded as an order-of-magnitude estimate; it can be visualized as the quantity of debris required to bury an area the size of a football field one mile deep.

The Oct. 20 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. Both avalanches contained a large percentage of ice, although the August 31 avalanche appears to have BEGUN as an ice avalanche, whereas the October 20 avalanche clearly began as a rock avalanche that subsequently scoured and entrained glacial ice. Neither avalanche was triggered by earthquake or volcanic activity.

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.

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