Debris Avalanches and Lahars at Mount Adams
The upper flanks of Mount Adams are steepened by glacial erosion, and some of the rock has been weakened by hydrothermal alteration, particularly on the west side. Consequently, slope failures and resulting debris avalanches are important processes at this volcano. Small debris avalanches commonly break loose from the cliffs overlooking Adams, Lava, Lyman, Wilson, Rusk, Klickitat, and Avalanche glaciers, and these avalanches typically incorporate varying proportions of ice and snow as they move downslope. Steep exposures of hydrothermally altered rock are especially prone to failure.
During the past 10,000 years, several considerably larger debris avalanches have occurred from Mount Adams. The largest Holocene debris avalanche at Mount Adams, which transformed into the Trout Lake lahar, originated high on the southwest side of the summit about 6,000 years ago. The massive (0.07 km3 or 0.02 mi3) landslide devastated a large portion (14 km2 or 5.4 mi2) of the volcano's southwest flank. Much of the material funneled into steep tributaries of the White Salmon River where it mixed with water and formed lahars that traveled downstream and spread out to cover about 15 km2 (8 mi2) of the Trout Lake lowland, impounding what became Trout Lake. A fraction of the material continued downstream leaving lahar deposits as far as Husum, 60 km (37 mi) from the source. Near Trout Lake, the deposit is as thick as 20 m (65 ft) and contains boulders up to several meters in diameter. Such a lahar would have had a catastrophic impact if it were to occur today.
About 300 years ago, another debris avalanche of weakened rock swept over a wide swath of the southwest flank of the edifice and sent a lahar into the upper Trout Lake lowland. Then again in 1921, about 4 million cubic meters (5 million cubic yards) of altered rock fell from the head of Avalanche Glacier on the southwest flank of the volcano and travelled almost 6 km (4 mi) down Salt Creek valley. This debris avalanche also incorporated sufficient water to partly transform into small lahars. Several similar but smaller debris avalanches have occurred from the head of Avalanche Glacier since 1921.