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.

A series of four lahars (volcanic debris flows and their deposits) occurred in rapid succession about 2,500 radiocarbon yr ago in the river system that drains the northwest sector of Mount St. Helens. The huge initial lahar had an instantaneous peak discharge near that of the Amazon River at flood stage, and the third in the series was the second largest in the history of the watershed. The deposits of the flows form a widespread terrace underlain by as much as 12 m of deposits, but its distribution reflects the fact that the lahars were the middle segments of flood waves beginning and ending as streamflow surges. These flood waves originated as lake-breakout surges analogous to those that would have been released, without engineering intervention, from the outlet-blocked Spirit Lake and several new lakes formed in 1980. The lahar deposits are granular and noncohesive, reflecting the origin of the entrained sediment as stream alluvium beyond the base of the volcano. Megaclasts consisting of blocks probably derived from an ancient debris avalanche are abundant in the initial flow and locally form a diamicton with the laharic diamicton as matrix. Although fine-pebble angularity resulting partly from cataclasis during flow is diagnostic of the lahars, a high content of rounded clasts and a channel facies with zones of clast support may explain why similar deposits have not been more widely recognized. The lahar channel facies superficially resembles alluvium. The depositional record of lahars and their distally evolved, hyperconcentrated lahar-runout flows many tens of kilometers from their source can provide a remarkably detailed history of volcanic activity. It may, in fact, provide the only evidence of some important events, of which the magnitude, frequency, and behavior are vital to the assessment of future volcanic hazards. Lake breakouts, meltwater surges caused by large pyroclastic flows, and forms of catastrophic volcanic ejection are examples of events that can create large downstream lahars but may leave little evidence of their true magnitude preserved on and near a volcano. The largest lahar in the river system, for example, began as a streamflow surge that bulked to a debris flow only after more than 20 km of flow.

Abstract There were two lahars that reached the Toutle River during the eruption of Mount St. Helens, Washington, on May 18,1980. The North Fork lahar was much larger than the South Fork lahar, had a much more rectangular hydrograph shape, and was much more destructive. Hydrographs (graphs of discharge versus time) constructed for both lahars demonstrate that differences between the lahars existed as close to the lahar sources as measurements were made, indicating that differences in processes that initiated the lahars must have been responsible for observed disparities between the lahars. The South Fork lahar was apparently generated when a laterally directed pyroclas-tic cloud triggered slab snow avalanches, and then rapidly incorporated and melted the snow. The North Fork lahar was generated from a small portion of avalanche debris in which ice was comminuted to an abnormally small size. This ice melted rapidly and saturated the host avalanche debris, which then liquefied during a long harmonic tremor event. The North Fork lahar differed greatly from the South Fork lahar because of significant dissimilarities between the pyroclastic cloud and harmonic tremor sequence that were directly responsible tor the characteristics of each lahar. Because differences in lahar characteristics were ultimately responsible for the contrast in destructiveness, I have concluded that the process of initiation is an extremely important factor controlling downchannel destruction. The importance of initiation must be accounted for in the quantitative analysis of lahar hazard.