DESCRIPTION:
Spirit Lake, Mount St. Helens, Washington

MSH80_blast_area_spirit_lake_with_rainier_10-04-80.jpg
Spirit Lake, once surrounded by lush forest, is within the area devastated by blast. Remnants of the forest float on the surface of the lake. Another Cascade volcano, Mount Rainier (14,410 feet [4,392 meters]), is in the distance. The view is from the south.
USGS Photograph taken on October 4, 1980, by Lyn Topinka.
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As a result of the May 18 eruption (May 18, 1980, eruption of Mount St. Helens) the source of the North Fork Toutle River was changed, and the upper part of its drainage system evolved on the dissected surface of a debris avalanche deposit. Prior to the May 18 eruption, the source of the North Fork Toutle River was Spirit Lake, which received runoff from 46 square kilometers of rugged, forested terrain, north of Mount St. Helens. The crater of Mount St. Helens was the source of the North Fork Toutle River from the time of the eruption until November 1982. ...

The outlet of Spirit Lake was blocked by the debris-avalanche deposit. During the period from May 18, 1980, until November 1982, the level of Spirit Lake rose as it received runoff from rain and snow in the surrounding mountains. The rising waters threatened to breach the unstable debris-avalanche deposits damming the lake. A temporary solution, put into effect by the U.S. Army Corps of Engineers, was the construction of a pumping station and pipeline near the southwest end of the lake until a more permanent solution could be devised. The pumping station went into operation in November 1982, creating a new source for the North Fork of the Toutle River. Water was released from the pump outlet at a constant rate of 5.0 cubic meters per second. It flowed across easily erodible volcanic deposits and caused rapid degradation and aggradation at cross-sections established downstream (Paine, 1984).

In May 1985, a tunnel was opened connecting Spirit Lake and South Coldwater Creek. Water was released from Spirit Lake into South Coldwater Creek at controlled rates of 5.7 cubic meters per second, 8.5 cubic meters per second, 11.3 cubic meters per second, and 14.2 cubic meters per second for approximately one-week periods during May 1985. As the level of the lake approached the tunnel gate elevation (1048.5 meters above NGVD of 1929) during June through August 1985, tunnel discharge declined from about 11.3 cubic meters per second to less than 5 cubic meters per second, the natural summer discharge from Spirit Lake.

The 1980 eruptions of Mount St. Helens in southwestern Washington marked the re-awakening of a relatively young (40,000 years) volcano that had been dormant since 1857. Frequent dacitic eruptions during the previous 2,500 years had produced pyroclastic flows, ash falls, debris flows, lava domes, and lava flows of andesite and basalt. Pyroclastic flows and lahars accompanied most eruptive periods and were largely responsible for forming fans around the base of the volcano, some of which dammed the North Fork Toutle River to form Spirit Lake between 3,300 and 4,000 years ago.

On May 18, 1980, part of the debris avalanche slid into Spirit Lake, raising its level nearly 60 meters and damming its natural outlet to a higher level. Water displaced by the avalanche surged up the surrounding hillslopes, washing the blown-down timber from the lateral blast into the lake. ...

The debris avalanche that triggered the eruption 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 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. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. ... The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet. ...

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. 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. ... As part of a long-term plan to cope with the continuing hydrologic hazards, the Corps of Engineers, in April 1985, completed the construction of a 1.5 mile-long (11 feet in diameter) diversionary tunnel at Spirit Lake. This permanent tunnel system replaced the temporary, barge-based pumping operations to regulate the lake's water level.

The four study lakes in the blast zone of Mount St. Helens (Spirit, St. Helens, Fawn, and Venus Lakes) received varying amounts of pyroclastic material, mud, ice, ash, and downed timber. The blast zone, as used in this report, is defined as the area immediately north of the volcano that was affected by the violent outburst, or explosion, that marked the beginning of the eruption. A detailed description of conditions in the blast zone at the time of the eruption was given by Korosec and others (1980).

Because of its proximity to the volcano, Spirit Lake was the most affected of the four lakes. A comparison of aerial photographs taken before and after the eruption shows that the lake increased in size and rose in altitude. The surface area of the lake increased from 1,300 acres to about 2,200 acres. Soundings of the east bay of the lake on October 16, 1980, indicated that the depth at the midpoint of the east bay, originally 190 feet (Figure 2), was less than 50 feet. Because of an extensive cover of logs on the surface of Spirit Lake, a post-eruption bathymetric map could not be constructed. The rise in lake level, estimated to be about 240 feet, is due to the blockage of the North Fork Toutle River, floods from melting snow on the volcano, and partial filling of the lake with debris (Youd and Wilson, 1980). Because of the blockage of the original outlet, surface outflow to North Fork Toutle River will not occur until the lake rises from its present altitude of about 3,440 feet to about 3,600 feet. The pre-eruption altitude of the lake was 3,198 feet.

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