Ancient Lake Bonneville Shoreline

June 15, 2015


Photographer: Rob Sheridan
Summary Author: Rob Sheridan
A pluvial lake is a landlocked basin filled with glacial meltwater and rainwater, historically associated with restricted outflow and increased inflow during periods of post-glacial warming. One of North America’s greatest pluvial lakes was Lake Bonneville that flooded the Great Basin 30,000 years ago. For the last 15,000 years, most of present-day Utah lay at the bottom of Lake Bonneville. This huge lake had over 2,000 mi (3,200 km) of shoreline and was 1,000 ft (305 m) deep in places.
About 14,500 years ago, the first of a series of abrupt drops in lake level occurred when Lake Bonneville overtopped a natural dam at the Red Rock Pass, in present-day Idaho. This resulted in a release of as much as 15 million cubic feet per second of water over a several month period into what is now the channel of the Snake River. A small island in the western part of Lake Bonneville represented the top of a tilted fault block. As the lake level lowered, more of this tilted block became exposed and is now known as Antelope Island. Over the next 5,000 years, other abrupt drops occurred, explaining the step-like appearance of the hills surrounding Salt Lake City and the slopes of Antelope Island.
Remnant lakes of the original Lake Bonneville can still be seen in three places: Sevier Lake, Utah Lake, and the Great Salt Lake. These three lakes represent the deepest parts of the original Lake Bonneville and are endorheic basins from which the major egress for water is now evaporation. This explains their very high salt content. The photo above shows a day-hiker (the author) viewing the remnant saline Great Salt Lake from one of the intermediate shorelines of Lake Bonneville on the slope of Antelope Island. Note the salt pans in the mid-ground. Photo taken in the summer of 2014.