The Crisis and Response
A magnitude 5+ earthquake at 8:32 a.m. set the blast in motion. Huge masses of rock began sliding down the mount's north flank, releasing gas pressure inside. Within seconds the whole north face exploded in a cloud of ash, rock fragments, and hot gases rushing northward at speeds up to 120 miles per hour. Once uncapped, the eruption continued vigorously until late afternoon. The top 1200 feet of the mountain rose in a billowing dark-gray plume of pulverized rock 11 miles high, and began drifting east.
Meanwhile, the avalanche of rock, mud and ice released by the earthquake roared down the mountain, turned westward and surged 17 miles down the North Fork Toutle river valley; one of the largest landslides in history. Another part of the slide pushed north across the valley, overtopped the ridge and flowed down South Coldwater canyon. The eastern part rammed into Spirit Lake, raising the lake level about 200 feet and blocking its outlet with debris hundreds of feet deep. Massive mudflows choked the Toutle and Cowlitz rivers and brought shipping to a halt on the Columbia River. The Corps, responsible for navigation and flood damage reduction, quickly became involved.
Corps crews found the worst flood danger on the Cowlitz along I-5, where the channel was nearly full of mud and debris. The Corps went to work raising levees and roads, removing debris and clearing blocked creeks between Castle Rock and Longview. At the same time, ships were stranded in port or going aground on mudflows that had come down the Cowlitz and filled the Columbia channel overnight from its previous 40-foot depth to a mere 14 feet. For nine miles up and downriver, nothing moved but mud and water. The Corps' three hoppers dredges and the Port of Portland's pipeline dredge soon arrived on the scene from other jobs. More pipeline dredges came on contract a few days later. When shipping was partially restored on the Columbia River, two of the huge dredges were hauled overland through Longview in the early morning hours to new work sites on the mud-choked Cowlitz.
During this emergency response period, feats of planning, fast and accurate decision-making and broad-scale coordination were going on behind the scenes. Through the summer, the Corps stepped up contracting procedures to clear the Cowlitz channel and reduce the flow of sediment before the winter storms set in. By November, contractors had built a low dam on each fork of the Toutle to keep more sediment from entering the Cowlitz and causing flooding downstream. Vast quantities of sediment were dredged from behind these structures and from the lower channel for the Cowlitz. Emergency dredging continued on the Columbia River until the ship channel was fully restored in late November.
Corps contractors also excavated basins on the Toutle to trap sediment, built and raised more levees along the Cowlitz, and dug outlet channels at several of the unstable new lakes created by the 17-mile blockage of the North Fork Toutle. Coordination with Tacoma City Light and the Bonneville Power Administration provided additional flood-storage at Mossyrock Reservoir. The winter passed without serious flooding-- but the mountain, too, had been very active, with smaller eruptions and new mudflows occurring every few weeks. Volcanic material from the avalanche continued to filter downstream. The work of stabilization had only just begun.
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