Abstract The 1980 eruption of Mount St. Helens caused instantaneous landscape disturbance on a grand scale. On 18 May 1980, an ensemble of volcanic processes, including a debris avalanche, a directed pyroclastic density current, voluminous lahars, and widespread tephra fall, abruptly altered landscape hydrology and geomorphology, and created distinctive disturbance zones having varying impacts on regional biota. Response to the geological and ecological disturbances has been varied and complex. In general, eruption-induced alterations in landscape hydrology and geomorphology led to enhanced stormflow discharge and sediment transport. Although the hydrolog-ical response to landscape perturbation has diminished, enhanced sediment transport persists in some basins. In the nearly 30 years since the eruption, 350 million (metric) tons of suspended sediment has been delivered from the Toutle River watershed to the Cowlitz River (roughly 40 times the average annual preeruption suspended-sediment discharge of the Columbia River). Such prodigious sediment loading has wreaked considerable socioeconomic havoc, causing significant channel aggradation and loss of flood conveyance capacity. significant and ongoing engineering efforts have been required to mitigate these problems. The overall biological evolution of the eruption-impacted landscape can be viewed in terms of a framework of survivor legacies. Despite appearances to the contrary, a surprising number of species survived the eruption, even in the most heavily devastated areas. With time, survivor “hotspots” have coalesced into larger patches, and have served as stepping stones for immigrant colonization. The importance of biological legacies will diminish with time, but the intertwined trajectories of geophysical and biological successions will influence the geological and biological responses to the 1980 eruption for decades to come.

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 km 2 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).