Lake Bonneville was a prehistoric lake that once dominated the landscape of the eastern Great Basin. At its maximum extent, this ancient lake was the size of Lake Michigan, stretching approximately 325 miles long and 135 miles wide. It reached depths of over 1,000 feet, transforming the arid region into a massive inland sea. Its formation during the last major global glaciation resulted from a unique combination of geological structure and dramatic climate shift.
Geological and Climatic Origins of Lake Bonneville
The formation of Lake Bonneville was enabled by the region’s distinctive basin-and-range topography. Tectonic forces created a closed drainage system, known as an endorheic basin, where water flows in but cannot flow out to the ocean. This bowl-shaped depression acted as a natural container, ready to be filled by a sustained increase in water supply.
The active trigger for filling this massive basin was the climate of the Late Pleistocene Epoch, often called the Ice Age. Beginning around 30,000 years ago, the regional climate became significantly cooler and wetter than it is today. This shift resulted in a pluvial lake, formed by a change in the water balance of the basin. Cooler temperatures dramatically reduced the rate of evaporation, while increased precipitation and runoff from surrounding mountains supplied a steady inflow of water.
This change meant that the amount of water entering the basin far exceeded the water lost through evaporation. While melting glaciers contributed some water, the vast majority came from direct rainfall, snowmelt, and increased river flow. The diversion of the Bear River into the Bonneville basin also significantly boosted the lake’s volume, providing a substantial, continuous source of fresh water. The basin continued to fill over thousands of years, with the lake level fluctuating as the climate oscillated.
The Physical Evidence: Shorelines and Terraces
The immense scale of Lake Bonneville is visibly etched into the surrounding mountainsides as distinct geological features. As the lake level rose and stabilized for extended periods, wave action and erosion carved ancient shorelines into the bedrock and deposited vast quantities of sediment. These features appear today as horizontal benches or terraces high above the modern valley floors.
The oldest prominent marker is the Stansbury shoreline, which formed during the initial rising phase of the lake. It represents a period when the lake covered approximately 9,300 square miles before continuing its ascent. The highest and most recognizable mark is the Bonneville shoreline, formed when the lake reached its peak elevation of approximately 5,090 feet above sea level around 17,500 years ago. This highest level marks the point where the water surface was controlled by the lowest pass in the basin rim, a natural dam of unconsolidated sediment.
A third significant feature is the Provo shoreline, a lower but equally prominent terrace. It formed after a catastrophic event dramatically lowered the lake level, which then stabilized at a new, lower elevation. These wave-cut benches provide a clear, measurable record of the lake’s former depth and volume. The sheer weight of the ancient water body also caused the Earth’s crust beneath the center of the lake to depress, a phenomenon known as isostatic loading.
The Great Drainage Event and Modern Legacy
The maximum stage of Lake Bonneville ended with the Bonneville Flood. Approximately 14,500 years ago, the peak water level at the Bonneville shoreline reached the lowest point of the basin rim at Red Rock Pass in southeastern Idaho. The water began to spill over the natural dam of alluvial fan deposits there, quickly causing the structure to fail.
This failure unleashed an immense torrent of water into the Snake River drainage system. The flood rapidly scoured a channel through the pass, dropping the lake level by over 350 feet and releasing an estimated 1,200 cubic miles of water. The event likely lasted only a few weeks to less than a year. The lake stabilized at the lower elevation of the Provo shoreline, which remained the controlling level for more than a thousand years.
As the Ice Age ended and the climate became warmer and drier, the lake began a prolonged recession phase. Today, the Great Salt Lake is the largest remnant of ancient Lake Bonneville, alongside smaller bodies like Utah Lake and the Bonneville Salt Flats. The Great Salt Lake is highly saline because it remains a terminal basin with no outlet to the ocean. Water entering from rivers carries dissolved salts, which are concentrated as fresh water is lost through evaporation.