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.

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.

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.

[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

[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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