Hebgen Dam is near Yellowstone National Park. In 1959 an earthquake with a magnitude of 7.5 occurred along a fault that crosses the Madison River. The quake damaged the dam, but not severely. The most spectacular and disastrous effect of the earthquake was the huge avalanche of rock, soil and trees that cascaded from the steep south wall of the Madison River Canyon. This slide formed a barrier that blocked the gorge and stopped the flow of the Madison River and, within a few weeks, creating Quake Lake almost 53 meters deep. The volume of material that blocked the Madison River below Hebgen Dam has been estimated at 28 - 33 million cubic meters. Most of the 28 deaths were caused by rockslides that covered the Rock Creek public campground on the Madison River, about 9.5 kilometers below Hebgen Dam. Madison Canyon Earthquake Area and Visitors Center offers interpretive programs about the 1959 earthquake. Click here for more photos.