Lake Bonneville

Lake Bonneville was a massive pluvial lake in western North America. A pluvial lake is a lake that forms because of increased precipitation and decreased evaporation. This lake existed during the Late Pleistocene, a period of major global glaciation. It occupied a hydrographically closed basin, also called an endorheic basin. This means the lake had no river outlets to carry water away.

Today, the Great Salt Lake, Utah Lake, and Sevier Lake are the largest remaining bodies of water in this basin. Lake Bonneville was much larger and deeper than these modern lakes.

The lake's size depended on a specific water balance. This balance is the relationship between water inputs and water outputs. Inputs include precipitation and runoff from rivers. Outputs include evaporation from the lake surface and evapotranspiration from the basin.

When inputs were greater than outputs, the lake level rose. This is called the transgressive phase. When outputs were greater than inputs, the lake level fell. This is called the regressive phase. Changes in global atmospheric circulation drove these climate shifts. These shifts altered the water budget of the entire Great Basin.

Lake Bonneville went through several distinct stages of growth and decline. It began rising about 30,000 years ago from levels similar to the modern Great Salt Lake. During its early transgressive phase, the level fluctuated slightly. Around 24,000 years ago, the lake began a rapid rise to higher elevations. It reached its maximum height about 17,500 years ago. This peak is known as the Bonneville shoreline. At this maximum, the lake covered an area nearly as large as modern Lake Michigan. Its shoreline was complex, featuring many islands and peninsulas.

Scientists identify three main shorelines that can be traced throughout the basin. These are named the Stansbury, Bonneville, and Provo shorelines. The Stansbury and Bonneville shorelines formed during the transgressive phase. The Provo shoreline formed during the overflowing phase. You can see these shorelines today as shelves or benches on mountainsides. They protrude from the mountains above the valley floor. These features are visible from long distances and even on satellite images. They consist of both depositional and erosional segments.

At its highest point, the lake reached the edge of its basin rim. This caused the water to overflow into the Snake River drainage near Red Rock Pass in Idaho. This overflow began as a trickle across a dam formed by the Marsh Creek alluvial fan. Eventually, it became a massive event called the Bonneville flood.

Groundwater sapping on the north slope of the fan made the dam unstable. The dam eventually collapsed, and the flood charged down the Marsh Creek valley. It flowed into the Portneuf River, then the Snake River, and finally the Columbia River. This flood likely lasted less than a year. It caused the lake level to drop by about 100 meters.

The study of Lake Bonneville has a long history. Early explorers noticed the ancient shorelines. Father Silvestre Velez de Escalante saw shell deposits in 1776. Captain John C. Frémont also recognized them in 1843. However, the geologist G.K. Gilbert provided the first major scientific description in 1890. He named the lake after Benjamin Louis Eulalie de Bonneville. Bonneville was a French-born U.S. Army officer and explorer. Gilbert's monumental work set the stage for all modern research on the paleolake.

Understanding Lake Bonneville helps scientists connect local history to global systems. While glaciers were expanding during this time, Lake Bonneville was not a proglacial lake. This means it was not formed directly by melting glaciers. In fact, mountain glaciers in the basin held less than 5% of the lake's maximum water volume. The lake also had no river connection to the massive Laurentide or Cordilleran ice sheets. Instead, the lake was a product of complex atmospheric and climatic changes. This makes it a vital example of how climate drives large-scale environmental shifts.