Lahars Sweep Down the Muddy River, Mount St. Helens

Within the first few minutes of the 18 May 1980 eruption of Mount St. Helens, numerous lahars were generated on the west, south, and east flanks of the volcano. One lahar originating on the upper east side of Mount St. Helens flowed 30 km down the Muddy River into a large reservoir in less than 30 minutes. At the base of the volcano, the Muddy River lahar flowed as an unchannelized broad sheet as fast as 110 km/hour. As the lahar flowed down the Muddy River valley, its velocity slowed to an estimated 10 to 20 km/hour and its depth varied from 2 to 9 m. The photographs below show some of the effects of the lahar as it traveled down the river valley.

Unchannelized Lahar, Headwater of the Muddy River
Photo: Pre-eruption view of Mount St. Helens Photo:Post-erution view of Muddy River near Mount St. Helens

Before Eruption

After Eruption

Channelized Lahar, Muddy River

Photo: Lahar damage in forest along Muddy River, MSH

Tree damage along Muddy River

Bridge on the Muddy River Destroyed by Lahar
Photo: Aerial view of destroyed bridge on Muddy River Photo: Ground view of bridge area along Muddy River

Aerial view of bridge destruction

Close up of same area

 

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URL of this document: http://volcanoes.usgs.gov/Hazards/Effects/MuddyLahar.html
Last modified: October 19, 1998

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