As an immediate effect of the slide, the water flowing over Hebgen Dam was stopped by the slide. The formation of a lake behind the slide began the moment of the slide. When it filled, this 240-foot deep impoundment, called Earthquake or “Quake” Lake, would exert an enormous pressure on the slide. If the slide was composed of unstable material, its collapse could, in a repeat of the Gros Ventre tragedy, bring death and destruction to the valley towns of Ennis, Three Forks, and Trident below.
The Hebgen slide on September 10, when Quake Lake first began to flow through the 14-ft. deep channel cut by the Army Corps of Engineers. The immense erosion that followed prompted another 50-ft. channel cut on the slide. The lower picture shows the completion of this project on October 17, 1959.(U. S. Army Corps of Engineers)
The second and greater threat was the discovery that when Quake Lake filled, its impounded water would lap at the foundations of Hebgen Dam, and quite possibly undermine it, releasing a volume of water seven times that of the earthquake-caused reservoirs, which could also sweep part of the slide along in its mad rush.
Like the threat of a time bomb, the rising level of Quake Lake, and the increasing pressure of the water against the slide, augmented by rumor, kept the downstream towns in constant anxiety.
The Army Corps of Engineers rushed into emergency action. They flew in a 50-man-staff, and set up headquarters in the Stagecoach Inn at West Yellowstone.
The mines in Butte were on strike, and huge earth moving equipment from the open pit operations, along with rigs from other contractors, worked around the clock to cut a 250-foot wide and 14-foot deep channel across the mile and a half long slide and armor it with rock so the water couldn’t cause sudden erosion of the rest of the slide before the water topped the huge natural dam.
On September 10, water licked over the new spillway, running into the river bed just below, which had been dry since the quake. To the Corps’s great surprise, severe erosion tore the downstream face of the slide. To remedy this, they launched another crash program to cut a 50-foot deep channel across the top of the slide. It was completed October 29.