Spirit Lake is a lake north of Mount St. Helens in Washington state. The lake was a popular tourist destination for many years until the 1980 eruption of Mount St. Helens. Prior to 1980, there were six camps on the shore of Spirit Lake: a Boy Scout camp (Columbia Pacific Council), a Girl Scout camp, two YMCA camps (Longview YMCA camp Loowit, and Portland YMCA camp), Harmony Fall Lodge, and another for the general public. There was also a number of lodges catering to visitors, including Spirit Lake Lodge and Mt. St. Helens Lodge; the latter was owned and operated by Harry R. Truman, who became entombed by the volcano's 1980 eruption.
During the 1980 eruption of Mount St. Helens, Spirit Lake received the full impact of the lateral blast from the volcano. The blast and the debris avalanche associated with this eruption temporarily displaced much of the lake from its bed and forced lake waters as a wave as much as 850 ft above lake level on the mountain slopes along the north shoreline of the lake. The debris avalanche deposited about 350,000 acre⋅ft of pyrolized trees, other plant material, volcanic ash, and volcanic debris of various origins into Spirit Lake. The deposition of this volcanic material decreased the lake volume by approximately 45,000 acre⋅ft. Lahar and pyroclastic flow deposits from the eruption blocked its natural pre-eruption outlet to the North Fork Toutle River valley at its outlet, raising the surface elevation of the lake by between 197 ft and 206 ft. The surface area of the lake was increased from 1,300 acres to about 2,200 acres and its maximum depth decreased from 190 ft to 110 ft. The eruption tore thousands of trees from the surrounding hillsides and swept them into Spirit Lake. These thousands of shattered trees formed a floating log raft on the lake surface that covered about 40% of the lake's surface after the eruption.
The water level of Spirit Lake is maintained at about 3,406 ft by draining water through Spirit Lake Outlet Tunnel, a gravity-feed tunnel completed in 1985. The 8,465-foot-long tunnel was cut through Harrys Ridge to South Coldwater Creek, which flows to Coldwater Lake and into the North Fork of the Toutle River. Had the lake level not been stabilized, the dam, which was composed of volcanic avalanche debris created by the 1980 eruption, would have been breached and caused catastrophic flooding within the Toutle River Valley.
