60th Anniversary of the Montana Yellowstone Earthquake: Quake Lake and the Hebgen Dam Incident

Background and History: Just before midnight on August 17th 1959 in southwest Montana, a M7.3 earthquake (4th largest recorded in the U.S. at that time) caused a 43 million cubic yard rockslide to cross the Madison River and up the opposite canyon. Rockslide debris (4000 feet long and 200 feet high) dammed the Madison River, forming Earthquake (Quake) Lake approximately 10 km downstream of Hebgen Dam. As water began rising behind the slide mass (made up of impervious weathered rock debris), officials feared that the landslide mass would be overtopped and breached or that water in Quake Lake would rise to the toe of Hebgen Dam, potentially destabilizing that dam. In either case, devastating flooding would likely occur downstream. Working around the clock, within two months, a spillway was constructed through the slide mass, lowering the lake to a safe level.