The Bonneville flood was a very large flooding event during the last ice age. It involved huge amounts of water covering parts of southern Idaho and eastern Washington along the Snake River. Unlike the Missoula Floods, which also happened during the same time in the Pacific Northwest, the Bonneville flood occurred only one time.
Cause and events
About 14,500 years ago (using a method called radiocarbon dating, this is 17,400 years in calendar time), pluvial Lake Bonneville in northern Utah reached its highest water level since it formed. The lake covered the area where the Great Salt Lake is today and was much larger, covering about 32,000 square miles (83,000 square kilometers). As the water level rose, it caused water to seep through and then overflow an ancient pass called Red Rock Pass, located at the start of the Portneuf River, a river that flows into the Snake River near the American Falls Reservoir. This pass was formed by two landforms called alluvial fans from opposite sides of a mountain notch, creating a natural dam. When the dam collapsed, it released a flood wave that was 410 feet high (120 meters) down the Portneuf River valley and also spilled into the nearby Bear River valley. When the flood reached the Snake River, it removed a lava dam at the site of today’s American Falls, releasing a lake called American Falls Lake that had formed behind the natural dam.
At the height of the flood, about 33,000,000 cubic feet of water per second (930,000 cubic meters per second) flowed over the Snake River Plain at speeds up to 70 miles per hour (110 kilometers per hour), carrying hundreds of cubic miles of dirt and rock from upstream. The flood wore away the 600-foot-deep (180-meter) Snake River Canyon through layers of basalt and loess soil, forming Shoshone Falls and other waterfalls along the Snake River. It also deepened and expanded many tributary canyons, including those of the Bruneau River and Salmon Falls Creek. The flood then entered Hells Canyon, making the gorge much wider. Eventually, the water flowed into the Pacific Ocean through the Columbia River.
Legacy
The flood at Red Rock Pass lasted only a few weeks, but erosion continued for several years until water stopped flowing over the pass. The flood removed the top 351 feet (107 meters) of Lake Bonneville, which held about 1,200 cubic miles (5,000 cubic kilometers) of water. This lowered the lake level to a stage called the Provo shoreline. The flood changed the Snake River Plain into a series of scablands with channels, similar to the Columbia Plateau. The flood also left behind many large boulders, called "melon" boulders, scattered throughout the canyons on the Snake River Plain. Some geologists believe the total amount of water from the Bonneville flood was greater than any single Missoula Flood. However, the Missoula Floods released more water overall, and at least one had a much higher peak flow rate. Much of the sediment moved by the flood was deposited near the mouth of the Snake River. Today, this sediment is covered by about 20 layers of deposits from the Missoula Floods.