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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.

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