• During the last Ice Age, a vast lake covered more than 25% of the total landmass of what is now the state of Utah, as well as parts of Nevada and Idaho. It was equal in size to the combined states of Vermont and Massachusetts. A dam of dirt and debris held the lake back at the location now called Red Rock Pass in Idaho.
• The lake covered 20,000 square miles and was over 1,000 feet (305 m) deep in places. If the entire shoreline were straightened into a line, it would reach from Red Rocks Pass in Idaho to New Orleans.
• Geologist Grove Karl Gilbert toured the area and was the first to officially hypothesize the existence of an ancient lake, publishing his findings in a report issued in 1890. He named the lake after Army Captain Benjamin Bonneville, who had explored the region previously and helped blaze the Oregon Trail.
• About 17,400 years ago, landslides caused by an earthquake fell into Lake Bonneville, resulting in a tsunami that jarred the dam loose. The water gushed downstream, encountering another large lake near American Falls, Idaho. The rush of water overcame a lava dam, and the combined torrents rushed forward in a wall of water over 400 feet (122 m) high.
• The resulting flood roared at a rate estimated to be three times the combined output of every river on the planet. It emptied an amount of water equal to the entire volume of Lake Michigan in just a few weeks. The deluge followed the path of the Snake River, then the Columbia River, all the way to the Pacific Ocean. Known as the Great Bonneville Flood, it rearranged much of the region’s topography.
• Shoshone Falls in Twin Falls, Idaho, was gouged by this massive flood, along with Hells Canyon.
• As the flood poured through the Snake River Canyon, it tore basalt boulders loose and carried them along. As the waters began to slow, the current dropped debris according to size: the largest car-sized boulders first, then mid-size rocks, then the smaller stones, and finally the gravels followed by sand.
• One area is littered with thousands of melon-size boulders. Some jokester put up a billboard advertising his gas station ahead, and he wrote on the back: “Petrified watermelons! Take one home to your mother-in-law!” After that, geologists began to refer to these stones as “melon gravels.”
• When the Great Bonneville Flood receded, it left a massive amount of sandy sediment near the current town of Mountain Home, Idaho. The wind swirled it into massive dunes in what is now Bruneau Dunes State Park.
• The dunes are enormous. The tallest is called the Big Dune and it’s 470 feet (143 m) high. By comparison, the tallest building in Idaho is the 8th & Main Building in Boise, which is 18 stories tall and reaches a height of 278 feet (85 m) (323 feet if you include the spire.)
• The Big Dune is not the biggest dune in the world (that’s the Duna Federico Kirbus in Argentina, which is 4,035 feet (1,230 m) tall, named after the researcher who measured it.) The Big Dune is not even the biggest in the U.S., which is Star Dune in Great Sand Dunes National Park in Colorado, at 755 feet (230 m). But Bruneau’s Big Dune is the largest “single-structure dune” in North America, a geological classification meaning that all the dunes in the area are connected to each other rather than being arranged in parallel rows.
• Today, only a small remnant of Lake Bonneville remains. It’s now called the Great Salt Lake in Utah. The Bonneville Salt Flats nearby are so completely flat that cars have raced there in speed trials since 1914.
