The Bonneville Flood, which occurred about 15,000 years ago, dropped the level of Lake Bonneville more than 300 feet to the Provo Level (4,740 feet above sea level). The 14,400-square-mile lake remained at this level for more than a thousand years, its level controlled by the spillover elevation at Red Rock Pass. It also was relatively fresh. Prominent deltas at the mouths of rivers entering the lake, and shoreline features such as spits, lagoons, and wave-cut benches mark this level. The University of Utah, Brigham Young University, Utah State University, and Weber State University campuses all are located on the Provo Level of Lake Bonneville. Were the lake to rise again in response to dramatically changed climate conditions, it could go no higher than this level because it, too, would flow out of the Great Basin into the Columbia River Basin at Red Rock Pass.
Approximately 12,000 years ago, the level of Lake Bonneville fell precipitously due to changes in the Great Basin climate. The Gilbert Level Shoreline ended about 10,000 years ago and left its mark about fifty feet above the present level of Great Salt Lake. It marks the last gasp of the Bonneville Lake cycle and the beginning of the story of Great Salt Lake.