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James B. Hale, Associated Press
In this photo taken Friday, Aug. 5, 2011, original access stairs are shown at the Teton Dam near St. Anthony, Idaho.

It’s been 40 years since the class I was teaching at a student ward of The Church of Jesus Christ of Latter-day Saints near Idaho State University was interrupted with an urgent request that we go home and change into work clothes.

The Teton Dam north of Rexburg, Idaho, had failed the day before, June 5, 1976, leaving a path of devastation in the flood that followed. The small cities of Teton, Idaho, Newdale, Idaho, and Sugar City, Idaho, were virtually wiped out.

The 15-foot wall of water that hit Rexburg hit a lumber yard where it lifted massive logs like match sticks. These logs turned into battering rams that smashed into bulk gasoline storage tanks that exploded, setting fire to the trees. The trees then smashed through buildings in the business district destroying hundreds by both fire and water. By the morning of Sunday, June 6, the leading edge of the wave had reached Idaho Falls, threatening the larger business district there, which is why we were dispatched to place sandbags along the beautiful falls that give the city its name.

How did this happen to a brand new dam? At 305 feet high and as wide as 10 football fields at the crest, the new U.S. Bureau of Reclamation dam was a dream come true for area farmers. The Teton River has unpredictable stream flows that some years flood and others years there is a drought. Containing that water behind a new earth-filled dam meant predictable water flows and electricity generation.

But controversy surrounded the dam. Area geologists testified that the site was incapable of supporting such a large structure because of the porous nature of the rock. Large caverns were found that had to be filled with concrete grout — more than twice as much as called for in the construction plans. But the USBR certified the site suitable and construction was completed in May 1976.

One of the problems that year was enormous rainfall in the area that filled the dam right up to the crest within a single season. It should have taken three years. The water level rose so quickly that the reservoir topped out before the primary outlet tubes were operational.

When a wet spot appeared two-thirds of the way up the face of the dam, managers called in emergency crews. Two Caterpillar tractors pushed additional fill material on top of the wet spot, but by then it was too late. Water at the front of the dam meant that the impermeable core was compromised. Incredibly, in just a matter of minutes, the dam swallowed up the two caterpillars while the frantic operators ran for their lives. At 11:55 a.m., the crest of the dam collapsed into a massive wall of water that quickly melted the dam into a torrent of muddy water. The 6-mile canyon below the dam became a raging torrent as the lake emptied.

More than a dozen communities and hundreds of farms lay in the flood’s path with tens of thousands of unsuspecting residents in peril. In a day before cellphones, it fell to radio, television and police cruisers with bullhorns to alert people to move to higher ground. Neighbors called neighbors, or drove to their homes to pick them up and drive them to safety.

Southeastern Idaho was originally settled by Mormon pioneers and home teaching and visiting teaching networks made it easier to get the word out quickly. Neighbors not on the calling tree were sought out and warned, too.

Tragically, 11 people died along with 13,000 head of livestock. But with more than 100,000 people in the path of the water, it was something of a miracle that more lives weren’t lost. Although Idaho Falls, Idaho, and Blackfoot, Idaho, suffered severe damage, the American Falls reservoir finally contained the flood, ending the disaster.

Perhaps the real story of the Teton Dam collapse was the remarkable relief efforts that supported the people and business owners whose homes and livelihoods were destroyed. Thousands of volunteers boarded buses in Utah, Idaho and Wyoming to help clean up the flooded areas. It was back-breaking work as they cleared away thousands of tons of contaminated mud and debris in the grueling summer heat. U.S. President Gerald R. Ford gave special praise to the volunteers when declaring the flood zone a disaster area.

As for those of us who placed sandbags in Idaho Falls, the river roared past with terrifying speed. When the water was just inches below the top of our sandbags we abandoned the effort, our work in vain. Today, nearly two-thirds of the Teton Dam remains while the Teton River winds harmlessly through the channel carved by the flood 40 years ago. It stands as a monument to both human error and resilience.

Sources: "Tragedy: A Chronology of the Teton Dam Disaster" (Bulletin Publishing Co, 1976); "Images of America: The Teton Dam Disaster" by Dylan J. McDonald (Arcadia Publishing, 2006); "Teton Dam History" on usbr.gov; The Em-Kayan Magazine, December 1974 and 1975 issues; Articles from the The Standard Journal on July 28, 1976, Aug. 11, 1976, and Sept. 7, 1976; an interview with Jim Roker, Rigby, Idaho, June 2015; "1976 Teton Dam Collapse" on bhs.idaho.gov.

Jerry Borrowman is a best-selling author of 15 published books. His newest release, "How 4 Feet of Plywood Saved the Grand Canyon" recounts the collapse of the Teton Dam and 7 other true stories from U.S. history. Visit him at www.jerryborrowman.com