The lake is the largest remnant of Lake Bonneville, a prehistoric pluvial lake that once covered much of western Utah. The three major tributaries to the lake, the Jordan, Weber, and Bear rivers together deposit around 1.1 million tons of minerals in the lake each year. As it is endorheic (has no outlet besides evaporation), it has very high salinity, far saltier than sea water, and its mineral content is constantly increasing. Its shallow, warm waters cause frequent, sometimes heavy lake-effect snows from late fall through spring.
Although it has been called "America's Dead Sea", the lake provides habitat for millions of native birds, brine shrimp, shorebirds, and waterfowl, including the largest staging population of Wilson's Phalarope in the world.
The Great Salt Lake is a remnant of a much larger prehistoric lake called Lake Bonneville. At its greatest extent, Lake Bonneville spanned 22,400 square miles (58,000 km2), nearly as large as present-day Lake Michigan, and roughly ten times the area of the Great Salt Lake today. Bonneville reached 923 ft (281 m) at its deepest point, and covered much of present-day Utah and small portions of Idaho and Nevada during the ice ages of the Pleistocene Epoch.
Lake Bonneville existed until about 16,800 years ago, when a large portion of the lake was released through the Red Rock Pass in Idaho. With the warming climate, the remaining lake began to dry, leaving the Great Salt Lake, Utah Lake, Sevier Lake, and Rush Lake behind.
The Great Salt Lake is fed by three major rivers and several minor streams. The three major rivers are each fed directly or indirectly from the Uinta Mountain range in northeastern Utah. The Bear River starts on the north slope of the Uintas and flows north past Bear Lake, into which some of Bear River's waters have been diverted via a man-made canal into the lake, but later empty back into the river by means of the Bear Lake Outlet. The river then turns south in southern Idaho and eventually flows into the northeast arm of the Great Salt Lake. The Weber River also starts on the north slope of the Uinta Mountains and flows into the east edge of the lake. The Jordan River does not receive its water directly from the Uintas, rather it flows from freshwater Utah Lake, which itself is fed primarily by the Provo River; the Provo River does originate in the Uintas, a few miles from the Weber and Bear. The Jordan flows from the north part of Utah Lake into the southeast corner of the Great Salt Lake.
A railroad line — the Lucin Cutoff — runs across the lake, crossing the southern end of Promontory Peninsula. The mostly solid causeway supporting the railway divides the lake into three portions: the northeast arm, northwest arm, and southern. This causeway prevents the normal mixing of the waters of the lake due to the fact that there are only three 100-foot (30 m) breaches. Because there are no rivers, with the exception of a few minor streams, flowing directly into the northwest arm, Gunnison Bay, it is now substantially saltier than the rest of the lake.
Categorically stating the number of islands is difficult, as the method used to determine what is an island is not necessarily the same in each source. Since the water level of the lake can vary greatly between years, what may be considered an island in a high water year may be considered a peninsula in another, or an island in a low water year may be covered during another year. According to the U.S. Department of the Interior and the U.S. Geological Survey, "there are eight named islands in the lake that have never been totally submerged during historic time. All have been connected to the mainland by exposed shoals during periods of low water." In addition to these eight islands, the lake also contains a number of rocks, reefs, or shoals that become fully or partially submerged at high water levels.