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Abstract

The dynamic landscape on the north flank of Mount St. Helens includes the largest debris avalanche deposit to accumulate within human history, covering more than 45 km2 on the upper North Fork of the Toutle River on 18 May 1980. Most land-forms on the debris avalanche are now relatively stable and only affected significantly by geomorphic processes exceeding certain energy thresholds. Following the debris avalanche, the most significant landscape-forming event has been the mudflow of 19 March 1982. This mudflow overtopped the rim of the largest explosion pit, formed and deepened channels, and largely formed the present landscape of the debris avalanche surface. Now that the power of geomorphic processes has diminished, only finer sediment is being moved. Channels are armored with coarser clasts, and valleys are plugging with sediment. Hikers can observe the new landscape from two selected overlooks. Johnston Ridge Observatory is the staging area for a recommended roundtrip hike of 13.6 km (8.4 mi).

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