It was a 305-foot high earthfill dam across the Teton River in the southeast part of the American state of Idaho. The dam was intended for irrigation, electricity, flood prevention, and recreation.
The dam was built by the United States Bureau of Reclamation, who of course got the blame for the collapse. It failed on Saturday, June 5, 1976.
The site is not closed off, but is not maintained at present writing (December, 2003). See references for directions on how to get there.
The dam is located in the eastern Snake River Plain, which is a broad tectonic depression on top of rhyolitic ash-flow tuff, late-Cenozoic volcanic rocks, dating to about 1.9 million years.
The tuff sits on top of sedimentary rock.
The area is very permeable, but no seepage was noted on the dam itself before June 5, though several springs had opened up downstream a few days before.
At the time of the collapse, the reservoir was filled with spring runoff. Water began seeping from the dam the Thursday before the collapse, not surprising for an earth dam.
The morning of June 5, a new leak appeared. Bulldozers were sent to plug it, but it grew. Local media appeared; an alarm was raised to downstream residents. Two bulldozers were caught in the eroding embankment, their operators pulled free by ropes. Around 11:57 AM, the dam collapsed. By evening it was drained.
Cities immediately downstream suffered horribly. The city of Idaho Falls had time to prepare, and the American Falls Dam, old an unstable, released water before the flood, but held, and the flood was over.
Cleaning up took the rest of the summer.
According to below Bureau of Reclamation site (references), "Today, Bureau of Reclamation engineers assess all Reclamation dams under strict criteria established by the Safety of Dams program. Each structure is periodically reviewed for resistance to seismic stability, internal faults and physical deterioration."
The usual recriminations flew (references), but as another of the below references observes, "Nature bats last."
Geology
The collapse
Aftermath
Lessons learned
External links and references