Hebgen Lake Earthquake and Quake Lake
With a full moon illuminating the countryside, vacationers who filled the resorts and campgrounds in the Hebgen Lake area the night of August 17,1959, were enjoying an especially pleasant and quiet summer evening.
Then at about 11:37 pm, the ground began to pitch and tremble; a major 7.5 earthquake was underway, and the mood of the night changed! Man-made structures moved, split apart and some collapsed. Folks, shaken awake, rushed outside wondering what had happened; stunned by the shock they could only stare in disbelief. The lake began to slosh back and forth like a tidal wave. The first few breakers, estimated to be 20 feet high, were large enough to flow over the dam. This seiche (“saysh”) action as it is called, continued for the next 11 1/2 hours.
A few miles downstream, in the Madison Canyon near the filled-to-capacity Rock Creek campground, part of the mountainside was shattered by a major tremor, and for a few brief seconds hung poised over the unsuspecting campers. Most assumed the shaking of their tents and trailers was caused by bears; to those who got up to investigate, in the clear moonlight everything seemed normal. Suddenly, the entire mountainside across the river began to fall away. As the campers tried to flee, the avalanche, with a terrible roaring and grinding noise, crushed and buried people, cars, and trailers. Survivors tell of being knocked down by a violent air blast and some were then engulfed by a huge wave of water displaced from the Madison River.
In the subsequent dust-filled stillness, the only sounds were the cries of the injured and the anxious calls of separated families.
As the night wore on, clouds gathered and masked the moon. What had begun as a cheerful summer night turned, in the space of seconds, into a time of terror, trauma and frustration. Those uninjured in the catastrophe, worked in the pitch-black darkness to rescue the trapped and wounded. The earth constantly trembled beneath their feet as repeated aftershocks shook the area and boulders and rock falls continued to tumble down the mountainsides. When daylight finally dawned, the immensity of the landslide became apparent.
About a mile-long section of the north-facing flank of a mountain, whose crest was about 1,300 feet above the river floor, had broken loose, and the entire mass moved across the canyon as a sheet. Estimates were it was moving about 100 mph when it engulfed the camp. The momentum of the slide was such that its front rode about 400 feet up the opposite canyon wall. The nearly 38 million cubic yards of rock that slid could have paved a three-foot-deep, two-lane highway from Montana to New York.
Once the valley was blocked, the waves that crested Hebgen Dam, plus the water, which is normally released through the spillway, were impounded east of the slide and Earthquake Lake, now known as Quake Lake, began to form.
Chilled and dazed, the fortunate survivors huddled on the slopes like war refugees, stranded with no way to call for help. Escape routes were blocked. Beside the enormous landslide sealing the lower end of the canyon, parts of the highway skirting the north edge of Hebgen Lake were either buried by landslides, under water or entirely broken up.
Communication lines were out. First reports of the disaster came from a ham radio operator in the badly damaged town of West Yellowstone, who had little knowledge of what happened in the Madison Canyon. All it was known for sure was that there was chaos, a threat of floods and trapped people desperately in need of help.
Before sunrise, rescue units for many agencies in Idaho and Montana were on the move. In the early morning light, a plane made a reconnaissance flight and Forest Service smoke jumpers parachuted into the canyon to give first aid and to set up communications. Men on foot, horseback and in helicopters moved in to give assistance. There were many outstanding acts of brotherhood and mutual help. Rescue workers treated the injured and evacuate them to nearby towns and ranches, where volunteers provided food and shelter. By evening a bulldozed road had been built; the immediate emergency was over, and all who wished to leave were able to get out of the canyon.
No one had known how many people were ensconced in and near the Rock Creek campground prior to the disaster and it would be a long time afterward until the toll on human life could be established.
Before all was stilled in the canyon and surrounding terrain, some 370 aftershocks were recorded. Twenty-eight people were killed, 19 of them entombed beneath the landslide.
After the quake, when Hebgen Lake quieted, it became apparent that the north shore had actually dropped 19 feet causing docks, beaches, and cabins to be submerged. By contrast, the south shore was elevated; stranded boats and docks were common sights and immense areas of lake bottom were exposed here. Near the lake, two parallel segments of the Hebgen Lake fault block broke the ground with one side stretching eight miles and rising 22 feet in places, creating a cliff-like earthquake scarp.
The 721-foot-long Hebgen Dam built in 1915, rises 87 feet above the river floor. The ground on which it rests dropped about 10 feet during the main tremor, this, and other events that night subjected the dam to stresses for which it was not designed. Though it did not fail, the structure was heavily damaged. The concrete core was cracked in at least four places and was tilted and twisted out of line.
The Corps of Engineers came in to study the landslide to prevent the rising waters of the newly created lake from cutting through the natural dam and flooding the downriver Madison Valley. By August 31, Quake Lake was five miles long, 150 feet deep and rising by nine feet daily. On September 9, only 22 days after the quake, a mile-long spillway was finished just as the waters were reaching the top of the dam. A huge flood was averted.
The epicenter of the quake was around Duck Creek on the east edge of Hebgen Lake; however, most of the damage moved in a westerly direction. The simple explanation for the cause of the earthquake is that huge fault blocks of rock, some 15 miles long, abruptly slumped and tilted beneath Hebgen Lake and the Madison River Canyon. To understand this process further requires a study of the underground hot spots and caldera activity in nearby Yellowstone National Park as it is all related. A fascinating book to read on this subject is Windows into The Earth by Robert B. Smith and Lee J. Siegel.
Yellowstone National Park itself shared in this night of terror, although not to the extent of the Madison Canyon. Out in the wilderness that night, in the Fire Hole River’s geyser basin, more-than-normal amounts of hot water started belching up. Numerous geysers, some dormant for many years, sent water skyward. Others erupted with more activity than usual. Old Faithful’s rhythm and force changed, and placid hot springs became angry, exploding with steam, water, and mud. When the earth finally began to shake, landslides obstructed some park roads.
In the Park’s venerable Old Faithful Inn, the evening’s festivities ended abruptly. Guests poured out of shaking buildings and thousands of cars, trucks and buses streamed out of the park in the face of the sharpest earthquake ever registered in the Rockies.
Today, hundreds of cormorants are often seen perched on the limbs of dead snags jutting out of Quake Lake along the highway. Perhaps the area is a resting and fishing stop on these large, black, bird’s migration route. A drive through the canyon will show the power of Mother Nature, as there are still signs of the damage done to the landscape. The US Forest Service administers a visitor’s center atop the rubble of the giant slide at Quake Lake. Called the Earthquake Lake Center, it is located on Hwy 287, about 50 miles south of Ennis, Montana.
The building is open from Memorial Day until mid-September. During the off-season, interpretive signs are reached by parking at the gate and walking up the road.
