Land & Resources Management

Spirit Lake Tunnel Intake Gate Replacement and Drilling Project

Click to view Spirit Lake Outlet project overview sheetThe Spirit Lake Tunnel Intake Gate Replacement and Geotechnical Drilling Project Decision Notice/Finding of No Significant Impact and Environmental Assessment, and related project information is available online at:

The Forest Service is directed to manage geologic hazards on National Forest System lands to ensure the protection of public safety, health, property and the environment. The Gifford Pinchot National Forest, in partnership with the U.S. Geological Survey and the U.S. Army Corps of Engineers, manage the outflow from Spirit Lake. The outflow consists of the Spirit Lake tunnel, intake gate, and debris blockage on the pumice plain.

Background: When Mount St. Helens erupted in 1980, a debris avalanche carrying 3 billion cubic yards of sediment formed a natural dam that blocked the outlet of Spirit Lake, where it had previously flowed into the North Fork of the Toutle River.  If this debris blockage were dislodged, an estimated 2.6 billion cubic yards of sediment would flow into the Toutle, Cowlitz and Columbia Rivers, causing massive damage and loss of life.

To assess possible lake control measures the U.S. Army Corps of Engineers (USACE) completed an Environmental Impact Statement in April 1984. The selected alternative was to construct the current tunnel through Harrys Ridge, which drains Spirit Lake water into South Coldwater Creek, flowing into the Nork Fork Toutle River. The tunnel is approximately 1.6-miles long and 11 feet in diameter.