The Long-Term Solutions
Spirit Lake and The Tunnel
One area of major concern was Spirit Lake. By 1982, its waters were rising dangerously high behind the debris dam left by the eruption. In May 1982, President Reagan asked the Corps to develop a comprehensive plan for long-term Mount St. Helens recovery. On August 19, the President declared a state of emergency at Spirit Lake, activating the Federal Emergency Management Agency (FEMA) to coordinate a full-scale federal response.
FEMA first asked the Corps to find an immediate, interim means of stabilizing the lake over the winter. The Corps installed barge-mounted pumps on Spirit Lake, and laid a five-foot-diameter outlet pipe two-thirds of a mile across the top of the debris dam. By November, this system had begun moving water out of the lake to a concrete stilling basin and from there into the North Fork Toutle River.
But a permanent solution still was needed. The Spirit Lake Decision Document, submitted by Corps planners in February 1984, recommended construction of a 8,460-foot tunnel to carry water through Harry's Ridge into South Coldwater Creek, thus maintain the lake at a safe elevation. Tunnel construction began in summer 1984 and was completed the following spring. Over the next few months, the tunnel was used to lower the lake level by about 20 feet. After predicted erosion along South Coldwater Creek during the initial drawdown period, the system proved itself effective in maintaining the lake's stability.
Spirit Lake Barge Mounted pump.
Erosion and the SRS
Corps planners were also concerned about long-term erosion from the debris avalanche in the upper North Fork Toutle River valley, where water was flowing through three billion cubic yards of volcanic material. Early estimates suggested that about one-third of that material (later revised down to 650 million cubic yards) could be expected to erode downstream in the decades following the eruption. Some of it would rattle along the streambeds and eventually reach the ocean. Some of it would float all the way through in the form of silt. But a large part of it would settle permanently in the river channels as sediments. All-out dredging efforts in the first year had removed just over 100 million cubic yards of rock and sediment from the Toutle, Cowlitz and Columbia Rivers. Dredging alone clearly was not the answer. Further efforts had to be made to stop the sediment from moving downstream.
A final Decision Document, approved in November 1985, recommended a three-part solution to this problem:
- Sediment Retention Structure [SRS] in the North Fork Toutle just upstream from the Green River confluence.
- A minor levee improvement at Kelso and supplemental dredging downstream.
- Dredging and levee reinforcement programs have been ongoing since the eruption, maintaining flood protection along the Cowlitz for the towns of Castle Rock, Lexington, Longview and Kelso. Hence the major new focus of long-term solution became the Sediment Retention Structure (SRS).
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