"Southern Idaho's Largest Flood in World History"
The Flood that Reshaped Southern Idaho
The Snake River Canyon is one of Idaho's most recognizable geologic features. Volcanic forces dating back more than 10 million years ago created the canyon. But it took the second largest flood in the history of the world to reshape it and to give the canyon its unique appearance as we see it today.
The Second Largest Flood in the World
Imagine standing here as a four hundred-foot tall wall of water racing at 177,000 cubic feet per second approached. The first hint of the advancing flood would have been gusts of wind, and the sound of a loud roar in the far distance. As the torrent raced through the canyon, it would have passed you traveling at nearly 100 miles per hour. More than likely you would have been standing here in water, as the entire canyon would have been submerged. The sound would have been deafening from the crash of boulders the size of a semi truck.
The Bonneville Flood, which lasted for nearly eight weeks, reshaped the entire length of the Snake River Canyon as it raced to the Pacific Ocean. Lake Bonneville once occupied one-third of Utah. Today the Great Salt Lake and the Bonneville Salt Flats are a vast landscape silently marking the site of one of the world's greatest glacial lakes.
Ancient Lake Bonneville
During the last Ice Age, prehistoric Lake Bonneville at 32,000 square miles (52,000 sq. kilometers) was larger in size than today's Lake Michigan. About 15,000 years ago an alluvial dam made of gravel located at Red Rock Pass near Pocatello, Idaho suddenly gave way rapidly draining Lake Bonneville into southern Idaho.
When farming began in southern Idaho near the Snake River during the early 1900's, farmers discovered a peculiar sight—rounded rocks scattered across the land. The rocks ranged in size from as small as a baseball to a half-buried boulder weighing hundreds of tons. Removing the rocks to clear the land for planting proved to be a daunting taks—quickly becoming a nuisance. The nuisance rocks, because of their round shape were humorously named: Idaho "petrified watermelons".
The "petrified melon" looking rock is river gravel deposited by the Lake Bonneville Flood. As the rocks traveled hundreds of miles in the fast moving torrent they became rounded much as rocks are rounded and deposited as gravel in creeks and rivers today. Where the floodwater slowed, it deposited its cargo of rocks across the land. Lake Bonneville Flood gravel is scattered across southern Idaho to the Oregon border.