The Great Salt Lake is the largest saltwater lake in the Western Hemisphere, and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah, and has a substantial impact upon the local climate, particularly through lake-effect snow. It is a remnant of Lake Bonneville, a prehistoric body of water that covered much of western Utah.
In an average year the lake measures approximately 1,700 square miles (4,400 km2), but its area can fluctuate substantially due to its low average depth of 16 feet (4.9 m). In the 1980s, it reached a historic high of 3,300 square miles (8,500 km2) and the West Desert Pumping Project was established to mitigate flooding by pumping water from the lake into the nearby desert. In 2021, after years of sustained drought and increased water diversion upstream of the lake, it fell to its lowest recorded area at 950 square miles (2,460 km²), falling below the previous low set in 1963.
Its three major tributaries, the Jordan, Weber, and Bear rivers together deposit around 1.1 million tons of minerals in the lake per year. Having no outlet besides evaporation, these minerals accumulate and give the lake very high salinity (far saltier than seawater) and density. This density causes swimming in the lake to feel similar to floating.
It has been called "America's Dead Sea", and provides a habitat for millions of native birds, brine shrimp, shorebirds, and waterfowl, including the largest staging population of Wilson's phalarope in the world.
The high salinity in parts of the lake makes them uninhabitable for all but a few species, including brine shrimp, brine flies, and several forms of algae. The brine flies have an estimated population of over one hundred billion and serve as the main source of food for many of the birds which migrate to the lake. However, the fresh- and salt-water wetlands along the eastern and northern edges of the Great Salt Lake provide critical habitat for millions of migratory shorebirds and waterfowl in western North America. These marshes account for approximately 75% of the wetlands in Utah. Some of the birds that depend on these marshes include: Wilson's phalarope, red-necked phalarope, American avocet, black-necked stilt, marbled godwit, snowy plover, western sandpiper, long-billed dowitcher, tundra swan, American white pelican, white-faced ibis, California gull, eared grebe, peregrine falcon, bald eagle, plus large populations of various ducks and geese.
There are twenty-seven private duck clubs, seven state waterfowl management areas, and a large federal bird refuge on the Great Salt Lake's shores. Wetland/wildlife management areas include the Bear River Migratory Bird Refuge; Gillmor Sanctuary; Great Salt Lake Shore lands Preserve; Salt Creek, Public Shooting Grounds, Harold Crane, Locomotive Springs, Ogden Bay, Timpie Springs, and Farmington Bay Waterfowl Management Areas.
Several islands in the lake provide critical nesting areas for various birds. Access to Hat, Gunnison, and Cub islands is strictly limited by the State of Utah in an effort to protect nesting colonies of American white pelican (Pelecanus erythrorhynchos). The islands within the Great Salt Lake also provide habitat for lizard and mammalian wildlife and a variety of plant species. Some species may have been extirpated from the islands. For example, a number of explorers who visited the area in the mid-1800s (e.g. Emmanuel Domenech, Howard Stansbury, Jules Rémy) noted an abundance of yellow-flowered "onions" on several of the islands, which they identified as Calochortus luteus. This species today occurs only in California, however, at that time the name C. luteus was applied to plants that later were named C. nuttallii A yellow-flowered Calochortus was first named as a variety of C. nuttallii but was later separated into a new species C. aureus. This species occurs in Utah today, though apparently no longer on the islands of the Great Salt Lake.
Because of the Great Salt Lake's high salinity, it has few fish, but they do occur in Bear River Bay and Farmington Bay when spring runoff brings fresh water into the lake. A few aquatic animals live in the lake's main basin, including centimeter-long brine shrimp (Artemia franciscana). Their tiny, hard-walled eggs or cysts (diameter about 200 micrometers) are harvested in quantity during the fall and early winter. They are fed to prawns in Asia, sold as novelty "Sea-Monkeys," sold either live or dehydrated in pet stores as a fish food, and used in testing of toxins, drugs, and other chemicals. There are also two species of brine fly as well as protozoa, rotifers, bacteria and algae.
Salinity differences between the sections of the lake separated by the railroad causeway result in significantly different biota. A phytoplankton community dominated by green algae or cyanobacteria (blue-green algae) tint the water south of the causeway a greenish color. North of the causeway, the lake is dominated by Dunaliella salina, a species of algae which releases beta-carotene, and the bacteria-like haloarchaea, which together give the water an unusual reddish or purplish color, and the bacteria converts non-toxic mercury into toxic methyl mercury, which then flows into the Southern portion of the lake in a heavy brine layer through the causeway.
Although brine shrimp can be found in the arm of the lake north of the causeway, studies conducted by the Utah Division of Wildlife Resources indicate that these are likely transient. Populations of brine shrimp are mostly restricted to the lake's south arm.
In the two bays that receive most of the lake's fresh water inflows, Bear River Bay and Farmington Bay, the diversity of organisms is much higher. Salinities in these bays can approach that of fresh water when the spring snow melt occurs, and this allows a variety of bacteria, algae and invertebrates to proliferate in the nutrient-rich water. The abundance of invertebrates such as gnat larvae (chironomids) and back swimmers (Trichocorixa) are fed upon extensively by the huge shorebird and waterfowl populations that utilize the lake. Fish in these bays are fed upon by diving terns and pelicans.