CS6B - Concurrent Session 06B: 1964 Blackfeet Flood and the Failures of Two Medicine and Swift Dams

In June 1964, northwestern Montana experienced one of the worst disasters in State history when a PMP-like event centered over the Blackfeet Indian Reservation. The result was widespread flooding, extensive damage, the failures of two high hazard potential dams, and tragically, the loss of 30 lives. As much as 14 to 16 inches of rain fell over a 36-hour period onto snow-packed watersheds within Glacier National Park and much of the Reservation, with the most severe rain and flooding occurring east of the Continental Divide. In what would become known as the Blackfeet Floods, rivers and reservoirs rose quickly as the rain-on-snow event resulted in runoff that far-exceeded flood stage in local waterways and overwhelmed the spillways of local dams. The Bureau of Reclamation’s Gibson Dam, for which this storm event is commonly named within the Dam Safety Community, was overtopped for 20 hours by depths as much as 3 feet, but the dam was able to withstand the extreme loading. Unfortunately, the same wasn’t true for Swift Dam and the Bureau of Indian Affairs’ Two Medicine Dam, which both failed during the event. Altogether the resulting floods affected an area of nearly 30,000 square miles, roughly equivalent to 20% of the state of Montana, with the most severe devastation concentrated on the Blackfeet Reservation. All 30 fatalities that occurred were on the Reservation, with 19 resulting from the failure of Swift Dam and 9 due to the extreme flooding discharged from the spillway of Two Medicine Dam prior to its failure. As part of the Decade Dam Failures series, this case study will summarize the many aspects of these tragic events that are instructive takeaways for the Dam Safety Community. In addition, this case study also expects to include elements of the Tribal perspective of these events and incorporate relevant firsthand accounts from Tribal individuals directly impacted by the floods.