The Great Salt Lake is a large hypersaline lake in northern Utah. It is the United States' largest natural lake west of the Mississippi River, the largest saltwater lake in the western hemisphere and the world's fourth-largest terminal lake. As a terminal lake, it is fed by four major rivers but possesses no outlet to the ocean. The rivers carry salt and other minerals to the lake, so the salt content of the lake is constantly increasing. As of 2011, the average salt concentration of the lake was about 12 percent — nearly four times saltier than the ocean — although the salinity of the lake can vary considerably, depending on fluctuations in the lake's volume.
During the last ice age, the current site of the Great Salt Lake was at the bottom of a much larger freshwater lake known as Lake Bonneville. Lake Bonneville had a surface area of about 20,000 square miles (about 51,800 square kilometers), compared with the Great Salt Lake's average surface area of 1,700 square miles (4,403 square kilometers). The climate in the Lake Bonneville area was much cooler and wetter than present-day Utah, which contributed to the volume of water in the lake. As the glaciers receded to the Arctic Circle, Lake Bonneville slowly evaporated, concentrating the salt and mineral content in the water to its modern level.
The Great Salt Lake is situated in a relatively flat basin, so the surface area of the lake fluctuates widely from year to year, depending on rainfall and rates of evaporation. Some stretches of shoreline might shift by as much as 15 miles (24.1 km) over the course of a few years. As a result, the lake is surrounded by extensive wetlands, which discourage development in the area and provide a habitat for many migratory birds. The lake's wetlands are such an important resource for birds that much of the area surrounding the lake consists of protected nature reserves.
The salt content of the Great Salt Lake is too great to support fish and most other marine organisms, leading to the lake's nickname of "America's Dead Sea." In spite of the extreme environment, brine shrimp and brine flies thrive in the salty water. The brine shrimp are an important economic resource and are harvested along with their eggs to manufacture fish food. Brine flies represent a key food source for birds. The lake's other inhabitants are limited to brine-tolerant algae and microscopic zooplankton.
The Great Salt Lake is located near Salt Lake City and provides the city with both a healthy tourism industry and a source of salt and other minerals. Salt companies extract nearly as much salt from the lake annually as the rivers deposit. Tourists are drawn to the lake by the unique scenery, the famously buoyant waters and the opportunity for a beach vacation in the middle of the desert.