Highway 287 plunged into Hebgen Lake along the scarp. Residents downstream feared that the dam would fail, causing the community of Ennis to evacuate shortly thereafter. Meanwhile, sizable waves, or seiches, spilled over the Hebgen Dam, threatening to fracture the dam’s already weakened infrastructure. Behind the slide, the Madison River flooded the canyon, engulfing trees and campgrounds alike and forming the beginnings of what is now known as Earthquake Lake. As debris plummeted to the canyon floor, a new natural dam formed at the foot of the canyon several miles downstream from Hebgen Dam. ![]() Shortly following the massive quake, an immense slab of weakly affixed material detached from the canyon’s north-facing slope and slid swiftly into the valley, snapping trees and tossing thousand-pound boulders in a fierce disarray. Within mere seconds, the ground plunged downwards along the fault line, leaving behind a sheer 20-foot wall of exposed earth. The initial quake lasted less than a minute. Just before the foot of the canyon sat Rock Creek Campground, a then-popular resting ground for tourists visiting Yellowstone National Park. At the foot of the canyon, the river dropped in elevation, entering the expansive Madison Valley. On either side of the timber-laden valley sat steep slopes coated with rocky outcroppings and dense forest. Prior to the quake, the Madison flowed from the dam outlet through a tucked-away canyon of the Madison range toward Ennis. Flowing both in and out of the lake is the Madison River, world renowned for its blue-ribbon fly-fishing. The Hebgen Dam, completed in 1914, sits at the head of the canyon barely out of sight of Highway 287. ![]() Today, the Madison River Canyon stretches for several miles west of Hebgen Lake, a large, manmade reservoir. It still is among the largest recorded in the United States, and the effects of the quake are still observed today. At its time, it was the second-largest recorded earthquake in the continental United States in the 20th century. 17, 1959, the 7.5-magnitude Hebgen Lake Earthquake hit southwest Montana. ![]() One late summer night, thousands awoke to a horrific nightmare: an enormous force rattling the ground beneath them.
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