Upgrading Hebgen Dam to Withstand a Potential Second Earthquake

Hebgen Dam has already survived a major earthquake … barely. On Aug. 17, 1959, a fault located only a few hundred yards from the right abutment of the earth embankment dam ruptured during a 7.5 magnitude earthquake. The Madison River downstream of the dam was completely blocked by a massive rockslide, which also killed 28 people. The dam held but was overtopped by a series of waves, and the dam and flow-control structures experienced severe damage.

The dam was repaired and operated well for the next few decades. Then, in 2008, several stoplogs in the intake structure failed, bringing the need to rehabilitate the intake to the top of the priority list for PPL Montana. The rehabilitation since undertaken, now nearly complete, should allow for reliable and safe operation of the intake for decades to come.