Lake Bonneville was the largest Late Pleistocene pluvial lake in the Great Basin of western North America. This vast body of fresh water formed approximately 30,000 years ago during the last glacial maximum due to cooler temperatures and increased precipitation that exceeded evaporation. At its peak, the lake covered an immense area of roughly 52,000 square kilometers, comparable to the size of Lake Michigan. It primarily occupied what is now western Utah but extended significantly into present-day Idaho and Nevada. The lake reached a maximum depth of over 300 meters, filling the topographic depression that today contains a much smaller, saline remnant.
The Threshold Breach: Initiating the Great Flood
The lake level rose steadily until it reached its highest elevation, known as the Bonneville shoreline, approximately 17,400 years ago. This maximum level was maintained for about a millennium because the water began to spill over the lowest point on the basin’s rim, located at Red Rock Pass in modern-day southeastern Idaho.
The natural dam at this pass was not composed of solid bedrock but rather of a thick accumulation of unconsolidated sediment, primarily an alluvial fan deposited by streams. This overflow was relatively stable, but the equilibrium was unsustainable as the flowing water began to erode the sediments and carbonates that formed the barrier.
The catastrophic drainage, known as the Bonneville Flood, was triggered by a rapid, massive failure of this natural dam. This failure was likely initiated by subsurface water seepage, a process called piping, which undermined the structure, or by overtopping, quickly eroding the soft materials.
The breach rapidly scoured a channel through the loose materials and down into the underlying rock layers, releasing a tremendous torrent. The event unleashed an estimated 4,750 to 5,000 cubic kilometers of water in a short period. This massive discharge lowered the lake’s surface elevation by approximately 107 meters, stabilizing the water level at a new, lower elevation called the Provo shoreline.
The Catastrophic Drainage Path
Once the threshold at Red Rock Pass failed, the escaping torrent immediately flowed north, following the natural contours of the land. The floodwaters first rushed into the Marsh Creek and Portneuf River valleys in Idaho, a surge estimated to be about 90 meters deep. This initial flow quickly joined the drainage of the Snake River near Pocatello, turning the existing river channel into a vast, temporary mega-river.
The sheer volume of the flood, reaching a peak discharge estimated at 930,000 cubic meters per second, was enough to scour the landscape down to bedrock. This peak flow rate was approximately 500 times greater than the maximum discharge ever recorded for the Snake River at Idaho Falls. Water traveling at speeds up to 110 kilometers per hour tore across the Snake River Plain, widening valleys and eroding away the soft loess soil.
The abrasive action created a terrain similar to the channeled scablands found in eastern Washington. The power of the water carved through the underlying basalt rock, deepening the Snake River Canyon by as much as 150 meters in some places. The flood created or enhanced several geological features, including Shoshone Falls near Twin Falls, Idaho.
The erosive force also moved massive “melon” boulders, some the size of automobiles, distributing them hundreds of kilometers downstream in the canyons of the Snake River Plain. The flood removed existing lava dams along the river, such as the one that had impounded American Falls Lake, releasing that water to join the main torrent.
The enormous flow entered Hells Canyon, a deep gorge on the border of Idaho and Oregon, significantly widening and deepening this chasm. The entire drainage event temporarily linked the Great Basin, which is normally a closed system, to the Pacific Ocean, allowing the water to exit the continent.
The ultimate destination of the floodwaters was the Columbia River drainage basin. The torrent poured into the Columbia River near the Tri-Cities area of Washington, combining its immense flow with the existing river, finally emptying into the Pacific Ocean.
Geological Legacy: Sculpting the Western Landscape
The catastrophic drainage left behind an indelible record etched into the mountainsides and the northern plains. The most visible remnants of the ancient lake are the massive, horizontal terraces, often called the “bathtub rings,” that wrap around the basin’s mountains. These prominent benches mark the prolonged stable water levels of the lake before and after the flood.
The highest and oldest of these is the Bonneville shoreline, which represents the lake’s level just before the breach at Red Rock Pass. The next major, widespread terrace is the Provo shoreline, which marks the stable elevation maintained for centuries after the main floodwaters had subsided. These shorelines now show slight variations in elevation due to the isostatic rebound of the Earth’s crust after the weight of the water was removed.
On the Snake River Plain, the flood permanently altered the topography, leaving behind extensive gravel deposits and exposing vast basalt formations. The abrasive action created a unique landscape of stripped rock and braided channels that contrast sharply with the surrounding terrain.
Today, the remnants of Lake Bonneville are represented by a few much smaller, disconnected water bodies. The largest of these is the Great Salt Lake in Utah, a highly saline terminal lake that occupies the lowest part of the ancient basin. Other survivors include freshwater Utah Lake and the ephemeral Sevier Lake to the southwest, all existing within the boundaries of the former mega-lake.