Background
Mount St. Helens erupted May 18, 1980, blasting more than 3 billion cubic yards of volcanic ash and debris 14 miles into the sky and thundering down the mountain in an immense landslide of mud and rock. The extraordinary natural disaster killed 57 people and impacted the lives of thousands more.
In the months following the eruption, the U.S. Army Corps of Engineers managed to offset the impacts of the flow of debris into the Toutle, Cowlitz and Columbia rivers. Congress assigned the Portland District the responsibility and authority to find long-term solutions to manage the continuing flow of sediment and reduce flooding. The Spirit Lake Tunnel, completed in 1985, helped stabilize the lake’s water levels. The Sediment Retention Structure, completed in 1989, keeps hundreds of millions of cubic yards of sediment from rushing down the Toutle River, preventing significant flooding and navigation problems.
Current actions
Today, sediment from the Mount St. Helens debris avalanche continues to cause flooding concerns to residents of Castle Rock, Kelso, Lexington and Longview, Wash.
In 2010, the Corps constructed a series of grade building structures designed to trap sediment flowing from the Mount St. Helens volcano, preventing it from making its way into the Cowlitz River system. Engineers believe these structures, located upstream from the SRS, combined with new modifications planned for the SRS spillway will help reduce the need for frequent dredging while still providing effective flood risk management.
The Corps is developing a long-term sediment management system designed to limit the amount of sediment flowing into the Toutle, Cowlitz and Columbia rivers. The long-term plan involves raising the SRS spillway in three 10-foot increments, up to 30 feet total. The plan also includes building dikes in the sediment plain above the SRS and dredging in the Cowlitz River as needed.
The first 10-foot spillway raise will form a shallow pool behind the SRS of about 2 million cubic yards and 290 acres. It is likely that sediment will fill to the elevated spillway crest within one to two years after construction. The second 10-foot spillway raise may occur 5 to 10 years after construction of the first 10-foot spillway raise. The spillway raise is expected to increase the overall trapping efficiency, because the raised crest will reduce the slope behind the SRS, which should cause more sediment to settle out before passing over the spillway.
The Corps is working with state and federal fish and wildlife agencies to ensure the Corps’ near-term sediment management actions will not impact downstream juvenile and adult fish passage, and ensure that long-term actions will provide the opportunity for upstream volitional fish passage in the North Fork Toutle River.