Spirit Lake (Washington)
|Location||Mount St. Helens National Volcanic Monument, Skamania County, Washington, USA|
|Primary inflows||Precipitation, streams|
|Primary outflows||Drainage tunnel|
|Basin countries||United States|
|Surface elevation||3,406 feet (1,038 m) (3,198 ft (975 m) before May 18, 1980)|
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. With the eruption and resulting megatsunami, thousands of trees were torn from the surrounding hillside after lake water was displaced 800 feet up the hillside. Lahar and pyroclastic flow deposits from the eruption then blocked the North Fork Toutle River valley at its outlet, raising the surface elevation of the lake by over 200 ft (60 m). The newly raised lake, once it reappeared, was also 10 percent smaller and much shallower.
After the eruption, Spirit Lake contained highly toxic water with volcanic gases seeping up from the lake bed. A month after the eruption, the bacteria-carrying water was devoid of oxygen. Scientists predicted that the lake would not recover quickly, but the reemergence of phytoplankton starting in 1983 began to restore oxygen levels. Amphibians such as frogs and salamanders recolonized the lake, and fish (reintroduced by fishermen) thrived.
Logs that were deposited in the lake during the Mount St. Helens eruption remain covering a large portion of the surface water. The rasping of logs together has deposited tree bark in the bottom of the lake, which form coal. The bare logs sink upright to the bottom of the lake due to the higher density of the root end, and land on layers of volcanic ash sediment, similar to petrified tree stumps. The high mineral content of the water rapidly petrifies the logs in upright position. Spirit lake is the first location where these processes of naturally occurring coal formation from log mat bark and rapid upright tree petrification were observed since being predicted by scientists shortly before the 1980 eruption.
The water level of Spirit Lake is maintained at about 3,406 feet (1,038 meters) by draining water through a gravity-feed tunnel completed in 1985. The 2,500-foot (762 meters) -long tunnel was cut through Harrys Ridge to South Coldwater Creek. Had the lake level not been stabilized, its mud dam created by the 1980 eruption probably would have been breached, possibly causing catastrophic floods in the Toutle River.
Before the eruption of Mount St. Helens, there were four camps on the shore of Spirit Lake: a Boy Scout camp, a Girl Scout camp, a YMCA camp, and another for the general public. There were also a number of lodges catering to visitors, including Spirit Lake Lodge and Mt. St. Helens Lodge; the latter was inhabited by Harry R. Truman, who became one of the volcano's victims.
In popular culture 
- NOVA-"Return to Mt. Helens" 1990
- Dale, Virginia H.; Swanson, Frederick J.; Crisafulli, Charles M. (2005). Dale, Virginia H.; Swanson, Frederick J.; Crisafulli, Charles M., eds. Ecological Responses to the 1980 Eruption of Mount St. Helens. doi:10.1007/0-387-28150-9. ISBN 978-0-387-23868-5.
- USGS Cascades Volcano Observatory: Spirit Lake
-  describes the tunnel that maintains the lake water level.