The largest of the pluvial lakes which formed in the Great Basin during the most recent Ice Age. Unlike other huge glacial lakes of the period, Lake Bonneville collected little glacial meltwater. Instead, the climate of western North America went through a "pluvial period", that is, one of much higher, more reliable rainfall than exists there today.
Lake Bonneville filled in the basins of northwest Utah and extended itself into northeastern Nevada and southeastern Idaho. Eventually, it extended for 19,691 square miles (51,000 square kilometers), the size of Lake Michigan or Costa Rica.
Somewhere between 15,000 and 14,000 years ago, the lake overflowed its area of internal drainage at Red Rock Pass, near Downey, Idaho. For several days, 15 million cubic feet of water poured through the pass every second, three times the flow of the Amazon today. This immense flood poured over the site of Pocatello, Idaho into the Snake River basin of Southern Idaho, eventually flowing into the Columbia River and eventually the Pacific Ocean. Even after the first few days, water poured through the pass at a reduced rate for up to a year, so that Red Rock Pass was carved down by 300 feet. It takes a flood of this magnitude to produce features such as "melon gravel" beds, piles of boulders from 3 to 10 feet (1-3m) in diameter. The Bonneville flood event was a principal shaper of the Snake River valley in Idaho today.
The other factor in Lake Bonneville's demise was a change in global climate: The Wisconsin Ice Age ended, and Utah's era of increased rainfall ended. Over the following millennia, the lake slowly dried up until only the Great Salt Lake, Utah Lake. and Idaho's Bear Lake remained. The Bonneville Salt Flats were formed when another region of the lake completely evaporated.
Unlike its more northerly cousin Lake Missoula, Lake Bonneville had only one catastrophic flood event. Sediments from the Lake Bonneville flood are layered between various Snake River Valley backfloods of Lake Missoula events (which produced the Channeled Scablands of Eastern Washington State).
Ancient shorelines of Lake Bonneville can be found in Utah's Wasatch Mountains, 934 feet above the level of the Great Salt Lake. Early explorer John C. Fremont noticed these shorelines and remarked that large, deep lakes must have existed at some time in the past. In the late 19th Century, G. K. Gilbert named the lake, and mapped its shorelines. He went on to show that the levels of the shorelines have been displaced by crustal movements in the Great Basin.
Idaho State University - Digital Atlas of Idaho - The Lake Bonneville Flood
Downey, Idaho official web site
Bureau of Land Management - Salt Lake Field Office - Bonneville Salt Flats
Utah History Encyclopedia - Lake Bonneville