Utah's Great Salt Lake and Ancient Lake Bonneville, PI39

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PI-39 Commonly Asked Questions About Utah's Great Salt Lake and Ancient Lake Bonneville

Where was Lake Bonneville, how large was it, and when did it exist?

Lake Bonneville was a large, ancient lake that existed from about 32 to 14 thousand years ago. It occupied the lowest, closed depression in the eastern Great Basin and at its largest extent covered about 20,000 square miles of western Utah and smaller portions of eastern Nevada and southern Idaho.

Outline of ancient Lake Bonneville and associated modern features.
(BSF = Bonneville Salt Flats).

What were Lake Bonneville's dimensions, and what was it like?

At its largest, Lake Bonneville was about 325 miles long, 135 miles wide, and had a maximum depth of over 1,000 feet. It contained many islands that are the present-day mountain ranges of western Utah. Its relatively fresh water was derived from direct precipitation, rivers, streams, and water from melting glaciers. During the time of Lake Bonneville, the climate was somewhat wetter and colder than now.

When and at what elevations were the terraces along the mountains made by Lake Bonneville?

Three major shorelines were left by Lake Bonneville, and one by the Great Salt Lake. The Provo and Bonneville shorelines of Lake Bonneville can be seen as terraces or benches along many mountains in western Utah. The Stansbury shoreline of Lake Bonneville and the Gilbert shoreline of the Great Salt Lake are less obvious, and are found lower in the valleys. Each shoreline represents an extended period during which the lake stood at that elevation. The four main terraces, their elevations, and the ages of their formation are given below.

Major shorelines of Lake Bonneville and the early Great Salt Lake
Shoreline Name	Elevation*** at Antelope Island	Approximate Age in Calendar Years before Present
Gilbert*	4,275	12,800 - 11,600
Provo**	4,840	16,800 - 16,200
Bonneville**	5,220	18,000 - 16,800
Stansbury**	4,445	24,400 - 23,200
* Great Salt Lake shoreline Lake Bonneville shoreline *Feet above mean sea level