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The Bonneville Floor

When the Teton Dam failed in 1976, it created a devastation torrent that killed several people and left millions of dollars of property damage.  Yet, it was barely a trickle compared to the Bonneville Flood.

About 14,000 years ago, Lake Bonneville covered much of Utah, Nevada and part of Southern Idaho.  It was really an inland sea, comparable to one of our modern Great Lakes.

As the lake level rose, the water began to spill over Red Rock Pass near Preston, Idaho.  The water eroded the pass until it cut through the hard rock and into a layer of softer soil.  Once the softer material was exposed, the flow became a flood.

The torrent crashed down the Portneuf River to where Pocatello is today, then sped downstream along the Snake.  In the 300-square mile Rupert Basin the water averaged 50 feet deep.  it filled the entire Snake River Canyon where Twin Falls and Shoshone Falls are today and still spread out across the Snake River Plain.

The flood ripped chunks from the walls of the canyon and carried boulders the size of cars along with it, rolling and polishing them as they tumbled.  Giant gravel bars 100 feet high and a mile long are common in the canyon.  There are hundreds of acres of melon-sized boulders left behind by the flood from Hagerman to Swan Falls.

At its peak, the Bonneville Flood rushed down the Snake River at a rate five or size times the flow of the Amazon.  About 600 cubic miles of water passed through idaho on its way to the Pacific, all in a matter of weeks.  The Bonneville Flood was the second largest in the geologic history of Earth