DESCRIPTION:
Castle Lake, Mount St. Helens, Washington

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.
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Castle Lake: Located in the Mount St. Helens National Volcanic Monument, Castle Lake requires a 20-plus mile drive over rough logging roads, followed by a tough hike down a steep hillside to the lake. The key to finding the lake is to first find the Weyerhaeuser 3000 road. Would-be anglers not familiar with the area should get a St. Helens West hunting map, published by the Washington Forest Protection Association. Those who put forth the effort have a chance to catch dandy-size rainbow trout; fish up to 10 pounds have been reported. This is a self-sustaining fishery, with no fish planted, so selective gear rules are in effect, along with a one-fish, 16-inch minimum size limit. The lake is open year-round, but the road in is usually blocked by snow until at least May.

When Mount St. Helens erupted in May, 1980, the debris flow that swept down the Toutle River blocked South Fork Castle Creek, impounding Castle Lake. In order to prevent the rising lake from overtopping the debris dam, a spillway was constructed in 1981 to stabilize the elevation of the lake, which now contains approximately 19,000 acre-feet (acre-ft) of water. If the natural dam were to fail, the ensuing breakout of Castle Lake might seriously affect communities downstream along the Toutle River. Although the dam and spillway have performed well to date, piezometers in the dam reveal hydraulic heads 30 feet or more above lake level in the dam crest. In this respect, the hydraulic head distribution in the natural dam does not resemble that in a engineered embankment dam.

A number of natural and human-made lakes exist close to the volcano in the North Fork Toutle and Lewis River valleys. ... Three natural lakes in the North Fork Toutle River, formed by natural debris dams during the 1980 eruption, have required modifications to their outlets in order to prevent catastrophic outbreaks. The U.S. Army Corps of Engineers provided (1) a tunnel outlet to Spirit Lake, (2) a bedrock spillway channel at Coldwater Lake, and (3) a reinforced spillway channel at Castle Lake to hold the levels of these lakes constant and to prevent them from overtopping their erodible natural dams. A recent study (Roeloffs, 1994), however, has verified earlier conclusions that the natural dam at Castle Lake is potentially susceptible to modes of failure other than overtopping and, under certain conditions, is only marginally stable. Castle Lake contains about 23 million cubic meters (30 million cubic yards) of water and would produce a large lahar if the blockage were to fail. We assume that an outbreak of Castle Lake is a potential hazard ...

The debris avalanche that triggered the eruption (of Mount St. Helens) slid north into Spirit Lake and west 25 kilometers down the North Fork Toutle River valley, covering the valley floor with unconsolidated debris to an average depth of 45 meters and as much as 180 meters in some places. ...

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. ...

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 of Coldwater and Castle Lakes, and water from Spirit Lake is currently being pumped into the Toutle River by the Corps of Engineers as a temporary measure to control its level. -- (Web note: a permanent tunnel opened on Spirit Lake in 1985)

... Part of the avalanche ( May 18, 1980 eruption of Mount St. Helens) surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. ...

By the fall of 1982, the debris dams for three of the largest lakes -- at Spirit Lake, Coldwater Creek, and South Fork Castle Creek -- were becoming substantially filled, thereby increasing the risk of catastrophic flooding should the dams fail or be overtopped. The Corps of Engineers, which in 1981 started construction of controlled outlets at Coldwater and Castle Lakes, began also to control the rise of the level in Spirit Lake by an interim plan of barge-based pumping and discharge into outlet channels -- (Web note: a permanent 1.5-mile-long tunnel opened in 1985). The USGS and the National Weather Service installed flood-warning systems in the Toutle and Cowlitz River Valleys. By March 1983, Spirit Lake contained 360,000 acre-feet of water, the lake at Coldwater had 67,000 acre-feet, and that at South Fork Castle Creek had 19,000 acre-feet. Scientists and engineers estimated that a breach of the natural dam at South Fork Castle Creek, the smallest of the three lakes, could unleash mudflows and floods comparable to those triggered by the May 18, 1980, eruption of Mount St. Helens. ...

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