The Great Salt Lake (GSL) in Utah is the largest saline lake in the Western Hemisphere. It is a shallow, hypersaline body of water, meaning its salt concentration is several times greater than that of the ocean. The lake undergoes dramatic fluctuations in its surface area, having varied between a historic high of about 3,300 square miles to a recent low of approximately 950 square miles. This constantly changing body of water is the concentrated remnant of a massive, ancient freshwater sea that once dominated the Great Basin landscape. Its history is a geological narrative written by ancient climates and a catastrophic flood event.
The Extent of Lake Bonneville
The modern Great Salt Lake is the final stage of a massive precursor known as Lake Bonneville, a colossal body of water that existed during the late Pleistocene Epoch. This immense pluvial lake began to grow significantly around 30,000 years ago, sustained by a regional climate that was both cooler and much wetter than today. Increased precipitation and reduced evaporation allowed water to collect across a vast geographical area, eventually filling the massive basin.
Lake Bonneville reached its maximum extent, known as the Bonneville shoreline, approximately 18,000 years ago. At this peak, the lake covered an estimated 20,000 square miles, stretching across western Utah and extending into parts of Idaho and Nevada. The depth of this ancient lake exceeded 1,000 feet in places, making it one of the largest late Pleistocene lakes in North America.
The shorelines of this massive lake are still visible today, etched into the surrounding mountain ranges, providing a clear visual record of its immense scale. The modern Great Salt Lake is one of the residual pools left behind as the climate changed.
The Geological Events That Shrank the Lake
The transformation from Lake Bonneville to the Great Salt Lake involved two distinct geological mechanisms: a rapid, catastrophic flood and prolonged climate-driven evaporation. The first significant change occurred around 17,400 years ago when the rising water level reached the elevation of the basin rim in southeastern Idaho. The lake overtopped the natural alluvial dam at Red Rock Pass.
This event triggered the catastrophic Bonneville Flood, which rapidly breached the dam and unleashed an estimated 1,000 cubic miles of water northward. The torrent of water lowered the lake level by over 350 feet in a relatively short period, stabilizing the lake at a new, lower elevation known as the Provo shoreline. This single event instantly reduced the lake’s size and depth.
Following the flood, the long-term, gradual shrinking was driven by a significant shift in regional climate. The end of the Pleistocene Ice Age brought a transition to the warmer, drier conditions of the Holocene Epoch. With less precipitation and glacial meltwater to replenish the basin, evaporation became the dominant force. This slow process accounts for the vast reduction in water volume, representing a decline of roughly two-thirds of the lake’s maximum depth.
The Hydrology Behind the Salt Concentration
The high salinity of the Great Salt Lake is a direct result of its unique hydrology as a terminal basin, also known as an endorheic basin. This means the lake has no natural outlet; water flows into the lake but can only leave through evaporation.
The three major tributary rivers—the Bear, Weber, and Jordan—flow into the lake, carrying dissolved minerals and salts weathered from the surrounding landscape. These rivers deposit millions of tons of dissolved solids into the lake each year. Since the water that evaporates is pure, leaving the dissolved solids behind, the mineral content becomes progressively more concentrated over time.
Conversely, the ancient Lake Bonneville was relatively fresh because it possessed an outlet at Red Rock Pass, allowing excess water and dissolved minerals to escape the basin. Once that outlet was lost due to the flood, the remaining water body became a closed system, setting the stage for the progressive accumulation of salt that defines the Great Salt Lake.