LONGVIEW — Authors of a new situation report on Mount St. Helens flood threats will present their draft study March 6 in Kelso.
Researchers for the Seattle-based Ruckelshaus Center have interviewed about 50 officials and citizens familiar with the mountain and its flood hazards to compile a report for the U.S. Forest Service.
They will present their findings and recommendations from 4 to 5:30 p.m. at the Cowlitz County Historical Museum, at the corner of Allen Street and Fourth Avenue in Kelso.
The Center is a joint effort of Washington State University and the University of Washington that seeks to encourage collaborative public policy in the Pacific Northwest. It is hosted and administered at WSU by WSU Extensionand hosted at UW by the Daniel J. Evans School of Public Policy and Governance.
The report chastised agencies around the region for not working more collaboratively to address flooding challenges and said decision-making should involve a broader array of groups and parties. It also emphasized the importance of thinking of Spirit Lake and the Toutle River as a system.
The volcano’s eruption on May 18, 1980, created two major flood threats:
It blocked Spirit Lake’s natural outlet into the north fork of the Toutle River, and geologists predicted the rising lake would eventually burst through the debris blockage and cause catastrophic flooding along the Toutle, Cowlitz and Columbia rivers.
By filling the headwaters of the valley with 3 billion cubic yards of erodible debris, the eruption created a massive and persistent sedimentation problem that even today threatens to clog the Cowlitz River and make Kelso, Longview, Castle Rock and Lexington more vulnerable to flooding.
The U.S. Army Corps of Engineers in the mid 1980s cut an 8,500-foot-long tunnel through Harrys Ridge to lower and stabilize Spirit Lake. But there is a long-term concern that a major earthquake could cause the tunnel to collapse in the area where it passes through a fault.
In the mid-1980s, the corps built a 125-foot-high sediment retaining dam to intercept the silt flow. The structure, maligned by some anglers and many residents of the Toutle Valley, has prevented hundreds of millions of cubic yards of sediment from flowing downriver. But it has largely filled up with debris and is passing silt downriver despite undergoing a slight raise to its spillway several years ago.
The corps last fall completed a long-awaited plan to address the continued flow of silt. It calls for raising the sediment-retaining dam another 23 feet, building other silt-retaining structures upstream of the dam and dredging the Cowlitz as necessary. It also will build a new fish trap below the dam.
Engineers acknowledged that the sediment flow will continue well into midcentury and may cost another $384 million through 2035 to keep Longview, Kelso, Castle Rock and Lexington safe.
Fisheries concerns delayed the plan, which took nearly a decade to complete. With such potential costs and the high stakes involved for public safety and fisheries resources, the Ruckelshaus Center says it hopes to create “a collaborative process to address challenges related to the long-term management of the Spirit Lake/Toutle-Cowlitz River system.”
