Dam Failure Case Study: Hebgen Dam (Montana, 1959)

Just before midnight on August 17, 1959, in southwest Montana, an Mw 7.3 earthquake caused a block of the earth's crust 125 square miles in area, including Hebgen Lake, to subside along established faults north of the reservoir. Up to 21 feet of vertical displacement was measured along the segment of the Hebgen Fault near the dam. The earthquake caused a massive rockslide to cross the Madison River and up the opposite canyon wall. Rockslide debris dammed the river, forming Quake Lake just downstream of Hebgen Dam. As water began rising behind the slide, officials feared that the mass would be overtopped and breached or that water in Quake Lake would rise to the toe of Hebgen Dam, potentially destabilizing that dam. Working around the clock, a discharge channel was excavated through the slide mass, lowering the lake to a safe level. Ground deformations from the earthquake also damaged Hebgen Dam, cracking the concrete core wall, damaging the spillway, and inducing a seiche on Hebgen Dam reservoir, causing waves to overtop the earthfill dam. The failure of the dam and uncontrolled release of Hebgen Lake would have been catastrophic for the downstream population at risk. Although severely damaged, Hebgen Dam did not fail and was later repaired.