USGS/Cascades Volcano Observatory, Vancouver, Washington
Mount St. Helens Hydrologic Activity
Debris Avalanche/Debris Flow of September 16-17, 1997
INFORMATION STATEMENT
U.S. Geological Survey and University of Washington Geophysics Program
Vancouver and Seattle, Washington
September 19, 1997
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Heavy rainstorms at Mount St. Helens on Sept 16-17 generated a
debris avalanche/
debris flow
from the crater of Mount St. Helens. Based on data
from monitoring instruments, the flow peaked at about 1:40 AM on September
17. Field investigations on the 18th indicate that a wet debris avalanche
about 50-80 feet thick passed over Loowit Falls. Most of the sediment was
deposited in the next mile of Loowit drainage. The majority of the flow
turned west at the base of the mountain and traveled downstream toward the
North Fork Toutle River, depositing most of the remaining sediment before
reaching the base of Johnston Ridge. The flow also sent a small amount of
sediment and water into Spirit Lake, washing out the hiking trail across the
Pumice Plain in at least two locations.
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The debris avalanche/flow was unrelated to any volcanic activity in the
crater. Compared to the volcanically-generated debris flows and debris
avalanches of the early 1980s, this event was small. However, this was the
largest storm-generated debris avalanche/flow from the crater in at least 10
years. Future rainstorms could generate similar events. USGS crews will
return to the crater today (September 19) to continue investigating the event.
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The U.S. Geological Survey and University of Washington Geophysics Program,
in cooperations with the U.S. Forest Service, continue to monitor conditions
at Mount St. Helens.
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[Image,216K,GIF]
Aerial view, Loowit Falls showing debris flow levee and location of
close-up photo.
-- USGS Photo by D. Dzurisin, September 22, 1997
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[Image,278K,GIF]
Close-up photo, debris avalanche levee of September 16-17, 1997, with USGS scientist for scale.
-- USGS Photo by J. E. Costa, September 17, 1997
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03/29/99, Lyn Topinka