Volcano landslides can dam tributary valleys

Coldwater Lake, blocked by landslide deposit, Mount St. Helens, Washington
Photograph by L. Topinka on January 13, 1984

View is looking northwest up the valley of former Coldwater Creek, now filled with a lake. When the landslide from Mount St. Helens slid into the North Fork Toutle River valley (foreground), it blocked the flow of Coldwater Creek. Water backed up behind the landslide deposit, gradually forming a lake about 8 km long and 55 m deep. The landslide was rushed down the Toutle valley from right to left.

Concern about the sudden breakout of water from Coldwater Lake from failure of the drebris dam or overtopping and subsequent erosion of the dam, led the Corps of Engineers in 1981 to control the lake level by excavating an outlet channel that delivers water to the Toutle River.

The fan-shaped delta on the southeast shore of Coldwater Lake forms where a stream from South Fork Coldwater Creek pours into the lake. The delta began to grow quickly when water from Spirit Lake was diverted into Coldwater Creek beginning in 1985. A long tunnel was drilled through a ridge to deliver water from Spirit Lake into South Fork Coldwater Creek in order to stabilize the level of Spirit Lake.

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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
URL http://volcanoes.usgs.gov/Imgs/Jpg/MSH/Coldwater1981_Topinka_caption.html
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Last modification: 16 December 1999 (SRB)