USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Natural Debris Dams and Debris-Dammed Lakes
- Natural Debris Dams
- Mount St. Helens, 1980
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MSH84_castle_lake_debris_avalanche_march_1984.jpg
The May 18, 1980 debris avalanche from Mount St. Helens covered over 24 square miles (62 square kilometers) of the upper Toutle River valley and blocked tributaries of the North Fork Toutle River. New lakes such as Castle Lake (pictured here) and Coldwater Lake were created.
USGS Photograph taken in March 1984 by Robert L. Schuster.
[medium size] ...
[large size]
From:
Wright and Pierson, 1992, Living With Volcanoes, The U.S. Geological
Survey's Volcano Hazards Program:
USGS Circular 1073
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In addition to the direct risk from
debris flows and
debris avalanches,
voluminous deposition of volcanic material in valleys commonly forms unstable
natural dams by blocking preexisting drainages. If the size and
structural integrity of the blockage is insufficient to hold back the reservoir
that will form (or withstand the erosion by overtopping flow), catastrophic
failure of the dam will result. Such a hazardous situation may persist for
months, years, or decades following an eruption. ...
From:
Brantley and Topinka, 1984, Volcanic Studies at the
U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory,
Vancouver, Washington:
Earthquake Information Bulletin, v.16, n.2,
March-April 1984
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The
debris avalanche
raised the level of
Spirit Lake
64 meters and dammed its natural outlet even higher.
Many small ponds filled closed depressions on top of the avalanche deposit,
and several lakes formed in tributaries dammed by the
avalanche; the largest lakes formed in the tributaries of
Coldwater and
Castle Creeks.
In late 1980, some of the ponds overtopped and swiftly eroded their new
outlets. The rapid release of water generated highly erosive flows on the
avalanche and transported large volumes of sediment down the lower reaches of
the Toutle and Cowlitz Rivers. ...
Failure of the debris dams holding
Spirit,
Coldwater, and
Castle Lakes
would result in catastrophic mudflows comparable to or larger than those
of May 18, 1980. Controlled outflow channels have been constructed to stabilize
the water levels ...
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On May 18, part of the debris avalanche slid into Spirit Lake,
raising its level nearly 60 meters and damming its natural outlet
to a higher level. ...
Tributaries of the North Fork Toutle River were dammed by the
massive debris avalanche deposit and subsequently formed lakes behind the
blockages. The largest of these are Coldwater and Castle Lakes.
Outlet channels were constructed in 1981 and 1982 to prevent the lakes
from overtopping their dams.
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Mount St. Helens May 18, 1980 Menu
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Mount St. Helens Lakes Menu
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02/22/05, Lyn Topinka