DESCRIPTION:
Mount St. Helens March 19, 1982 Lahar

[Image,73K,JPG]
An explosive eruption at Mount St. Helens on March 19, 1982, sent pumice and ash 9 miles (14 kilometers) into the air, and resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley. Part of the lahar entered Spirit Lake (lower left corner) but most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 50 miles (80 kilometers) downstream.
-- USGS Photo by Thomas J. Casadevall, March 21, 1982

A large volume of snow and ice is presently accumulating in the Mount St. Helens crater, protected by the shade of the high, steep crater walls. This accumulation provides a growing potential water source for lahars in the North Fork Toutle River valley. It is already mixed with rock debris eroded from the crater walls, and this debris would augment the formation of a lahar. It is possible that a large eruption could melt most or all of this snow and ice in a matter of tens of minutes. A very small eruption in 1982 rapidly melted enough snow and ice in the crater to trigger a 4 million cubic meters (5.2 million cubic yards) flood that transformed into a lahar and flowed all the way to the Cowlitz River. ...

An explosive eruption of Mount St. Helens on March 19, 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack. A blast of hot pumice, dome rocks, and gas dislodged crater-wall snow that avalanched through the crater and down the north flank. Snow in the crater swiftly melted to form a transient lake, from which a destructive flood and lahar swept down the north flank and the North Fork Toutle River.

Since the explosive eruptions of Mount St. Helens in 1980, dacitic lava has erupted periodically to build a dome that in early 1982 rose some 200 meters above the crater floor. On March 19, 1982, a dome-building eruption began with an explosive phase. Had the eruption occurred in summer, the destructive effects of the relatively small explosive eruption would have been confined to the crater and the upper flanks of the volcano. But because snow thickly mantled the steep crater wall, a lateral blast generated a large avalanche that flowed 8 kilometers off the volcano. Heat from eruption products meanwhile swiftly melted snow, producing a transient lake whose sudden discharge extended effects of the eruption far downvalley. The complex sequence of eruptive and flow events -- probably all occurred within a few minutes -- are inferred from stratigraphic field relaitons gathered within a few days of the eruption and from geophysical data and distant visual observations. ...

... Beyond 35 kilometers from the crater, channels aggraded, in contrast to the notable downcutting that occurred upvalley. By the time the flow reached the Cowlitz River confluence 84 kilometers from the crater, it was so dilute that it produced only a 25-centimeter rise in the river at a nearby gaging station.

Peak discharge was an order of magnitude less than that of lahars that had devastated the Toutle River Valley on 18 May 1980. Nonetheless, the 1982 lahar breached and severely eroded a debris-retention dam 35 kilometers from the crater and caused extensive deposition throughout the lower Toutle River. Mudlines of March 19-20, 1982, were only a few meters below those of the great May 18, 1980, lahar, because aggradation during winter storms in 1981 and 1982 had severely reduced channel capacity. As long as Mount St. Helens remains active, a potential exists for large snow avalanches, floods, and lahars to be destructive far from the vent.

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