Lake Bonneville was an ancient lake that covered much of present-day Utah, in the western United States. It had roughly the same area and depth as present-day Lake Michigan. Much of Lake Bonneville drained thousands of years ago. Further evaporation reduced the lake to a number of smaller, isolated lakes. The Great Salt Lake is the largest of the remaining lakes. Lake Bonneville was named after Benjamin L. E. de Bonneville, who organized explorations to the region in the 1830’s.
Scientists believe that lakes have existed on the site of the Great Salt Lake for millions of years. Lake Bonneville formed more than 30,000 years ago, fed by streams, rainfall, and glacial runoff. Starting about 28,000 years ago, the lake grew to occupy much of the eastern part of the Great Basin region. See Great Basin .
Eventually, Lake Bonneville’s eastern shores reached the mountains of the Wasatch Range. Mountain peaks throughout western Utah formed islands within the lake. Evidence of the ancient shore of Lake Bonneville can be found in these mountains at about 5,000 feet (1,520 meters) above sea level.
A natural alluvial dam existed at the north end of Lake Bonneville, near what is now Zenda, Idaho. Alluvial dams are made of loose deposits of gravel, silt, and rock. As lake levels rose, water began eroding the dam. About 14,500 years ago, the dam failed catastrophically, lowering the outlet by more than 350 feet (107 meters) and flooding the Snake River Plain.
Lake Bonneville became shallower as a result of the flood. The lake stabilized at the level of Red Rock Pass, a mountain gap a short distance south of the Zenda outlet. The lake’s post-flood level—about 4,700 feet (1,430 meters)—is marked by what geologists call the Provo shoreline.
From about 12,500 to 11,000 years ago, Lake Bonneville shrank significantly as this region experienced warmer, drier weather. Evaporation from the lake increased, and the lake became shallower and more _saline—_that is, higher in salt content.
Today, the Great Salt Lake, freshwater Utah Lake, and the usually dry Sevier Lake are the largest remnants of Lake Bonneville. The Bonneville Salt Flats, an area of level salt beds in the Great Salt Lake Desert, gained fame as a site for automobile racers to set land speed records.
