Spirit Lake

The outlet of Spirit Lake was blocked by Mount St. Helens' May 18, 1980, 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.

Spirit Lake Tunnel

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.

-- Excerpts from: Meyer and Dodge, 1988, Post-Eruption Changes in Channel Geometry of Streams in the Toutle River Drainage Basin, 1983-85, Mount St. Helens, Washington: USGS Open-File Report 87-549, 226p.