Lake Bonneville was a colossal prehistoric lake that once dominated the Great Basin of western North America during the late Pleistocene Epoch. Formed under the influence of a cooler, wetter climate, it covered approximately 52,000 square kilometers (20,000 square miles), comparable in size to modern Lake Michigan, reshaping the landscape of what is now Utah, Idaho, and Nevada. Its disappearance involved both sudden, catastrophic draining and slow, relentless evaporation, leaving behind the smaller, highly saline remnants seen today.
When Lake Bonneville Reached Its Maximum Size
Lake Bonneville began forming roughly 30,000 years ago as the climate shifted toward glacial conditions, increasing precipitation and reducing evaporation. A sustained rise started around 24,000 years ago, reaching its greatest extent during the Bonneville highstand, approximately 18,000 to 17,500 calendar years ago.
At its maximum, the lake reached an elevation of about 1,552 meters (5,090 feet) above sea level, known as the Bonneville Shoreline. This highstand was defined by the lowest natural pass in the basin: an alluvial fan at Red Rock Pass in Idaho. Sustained by glacial meltwater and runoff, the deep freshwater body covered roughly a third of modern Utah.
This equilibrium was maintained for a period, with water beginning to slowly spill over the Red Rock Pass threshold into the Snake River system. The high stand represents the peak of the lake’s existence, controlled entirely by this overflow point. The extensive, ancient shorelines visible on the mountainsides are a lasting geological signature of this maximum size.
The Catastrophic Bonneville Flood
The dramatic reduction in Lake Bonneville’s volume was not gradual but a single, massive event known as the Bonneville Flood. This catastrophic outburst occurred around 17,400 calendar years ago, when water overtopped the natural dam of unconsolidated sediment at Red Rock Pass. The overflow quickly eroded the earthen barrier, initiating an immense torrent of water.
The flood released an estimated 4,750 to 5,000 cubic kilometers (1,200 cubic miles) of water over a period of weeks. This discharge was one of the largest known floods in Earth’s history, scouring the landscape along the Snake River Plain. Peak discharge was estimated at 930,000 cubic meters per second.
The immediate effect of this breach was a rapid drop in the lake’s surface elevation by more than 100 meters (350 feet). The water level stabilized at a new, lower elevation defined by a bedrock sill at Red Rock Pass, forming the Provo Shoreline. This event permanently reduced the lake’s volume by about one-third.
The Final Evaporation and Formation of Modern Lakes
Following the catastrophic flood, Lake Bonneville remained at the Provo Shoreline level for an estimated 600 to 2,000 years. The long-term process of the lake truly “drying up” began as the Pleistocene Ice Age ended and the climate became significantly warmer and drier. This shift led to a negative water balance where evaporation exceeded inflow.
The lake’s final regression, driven by this climate change, started in earnest around 16,000 to 13,000 years ago. Over the next few thousand years, the water level gradually declined about 200 meters (660 feet) to near-modern levels. This slow, continuous process of evaporation concentrated the dissolved salts within the remaining water.
A brief, final minor highstand, known as the Gilbert Level, occurred between 11,000 and 10,000 years ago, marking the final stage of the Lake Bonneville cycle. After this, the water continued to retreat into the lowest depressions of the basin. The lake was essentially gone in its massive form by about 13,000 years ago, leaving behind a series of smaller, disconnected bodies of water.
The most prominent remnant is the Great Salt Lake, a terminal lake where inflow is balanced only by evaporation, resulting in its high salinity. Other surviving remnants include the freshwater Utah Lake and the saline Sevier Lake. The vast, flat salt beds, such as the Bonneville Salt Flats, are a direct result of this final desiccation process.