Great Salt Lake, Utah

Great Salt Lake, Utah, is a perennial water body and has been so for the last 30,000 years. It is the largest saline lake in the United States and one of the largest perennial saline lakes in the world, measuring some 120 km long and 55 km wide, with a water surface elevation that oscillates around 1,280 m above sea level. At this elevation, the lake brines cover an area of 4,180 km2 and have a maximum depth of 10 m. In a typical year, the water surface fluctuates around 30–60 cm, reaching its highest level in May-July (after snowmelt) and dropping to its lowest level in October-November.

Since 1847 the water level of Great Salt Lake has varied from a low of 1,277.5 m in 1963 to a high of 1,283.75 m in 1986–87. The 3.7-m rise in the level of the Great Salt Lake between 1982 and 1986, leading to the record historical high in lake water level, was at least partly related to the record rainfall and snowfall in its catchment during the 1982/3 El Niño event. Because the lake is so shallow and its floor so flat, small changes in water level correspond to massive migrations of the strandline. Likewise, the salinity of the main lake water body varies and, depending on the lake's water level, ranges from 50 to 270 ‰.

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Great Salt Lake, Utah. The lake is subject to large changes in the extent of its perennial water body. The upper figure shows the different extent of its highest and lowest levels in historical time. The chart plots yearly changes of lake water level since 1847.

The Great Salt Lake lies in an endorheic basin with no surface outlet. The main rivers entering the lake are the Bear River from the north, the Weber/Ogden River from the east, and the Jordan River from the south (Jones et al. 2009). Together with small volumes of bajada runoff, the rivers supply around two-thirds of the water currently entering the lake. Direct precipitation into the lake supplies some 31%, while groundwater supplies the remaining 3 %. The drainage basin of the lake today covers an area of about 57,000 km2. Water is lost from the lake mostly through evaporation, with something like 3,500 km3 of water evaporating annually. When inflow equals evaporation, the level of the lake remains constant. If the inflow is higher and less than evaporation, the level of the lake will rise or fall, respectively.

Being situated along the Wasatch Front at the eastern edge of the tectonically active Basin and Range Province means the size, shape, and location of the Great Salt Lake depression has been a response to variable intensities of subsidence and faulting, overprinted by the intense climatic vagaries of the Late Pleistocene. The lake currently straddles BSk and Cfa Köppen climate zones. The present-day Great Salt Lake is a remnant of a much larger and fresher water body that was once linked into the Pleistocene Lake Bonneville system, as were Utah Lake, Sevier Lake, and the Bonneville Salt Flats. Lake Bonneville existed until about 16,800 years ago when a large portion of the lake waters was released as a mega-flood through Red Rock Pass in Idaho. This was a catastrophic event with an estimated maximum discharge of 420,000 m³/s sec, or about three times the average flow of the Amazon River. The speed of flow peak flow was around 7 m/sec although this peak likely lasted only a few days, voluminous discharges may have continued for at least a year.

The former areas of perennial fluvial inflow into Pleistocene Great Salt Lake are defined by classic "bird' s-foot" or "Gilbert-style" hanging deltas. The downcutting of the current hydrology and the presence of Pleistocene shoreline terraces on higher levels of the delta prism suggest deltas were most active during Pleistocene lake full stages (Lake Bonneville time) when the lake was not accumulating evaporite salts. At the time the mega lake's deepest water depth was ≈ 280 m compared to a maximum depth that fluctuates around 7-10 m today.

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Sediment distribution in the Great Salt Lake and surrounds. A) Distribution of depositional settings in the Bonneville/Great Salt Lake basin, Utah. The lake itself is a perennial water body fed by rivers with “bird’s-foot” deltas at the older entry points into the lakes. The saline pan facies are halite-rich with lesser glauberite, while the dry and saline mudflats are subaerially exposed parts of the old subaqueous floor of Lake Bonneville. Carbonate mudstones, mostly aragonite pellets, dominate sediments currently accumulating on the subaqueous lake floor (after Smoot and Lowenstein, 1991). B) Distribution of algal tufa reefs and ooid sand shoals about the strandzone at the edge of the perennial water body in the Great Salt Lake. Positions of the East Lake Fault (ELF) and the Carrington Island Fault (CF) are shown (after Eardley, 1938; Colman et al., 2002). C) Schematic cross-section based on seismic lines between Antelope Island and Carrington Island showing the importance of syndepositional faulting in controlling the thickness of post-Bonneville (<13.5 ka) sedimentary packages (grey) and the distribution of the mirabilite bed in the lake (after Colman et al., 2002). 

Modern strandzone deposits are mostly shorezone spits and bars, which are dominated by aragonitic ooid and peloidal sands (mostly brine shrimp pellets). Carbonates also precipitate about the lake strandline in spring mounds, algal mounds, and stromatolites. Older algal reef mounds are located further out in the lake and are now buried by lake muds; they grew earlier in the Holocene when lake water levels were lower than today. Oolite shoals today form sand waves that define the lake strandzone. Individual ooids are aragonite with a radial crystal structure. When and how the individual ooids grew is not well understood, but halobacteria and organic matrices are thought to play a role. Laminated mudstones, in large part composed of the faecal pellets of brine shrimp, are currently accumulating further out in the lake. The pellets are a mixture of aragonite and detrital matter, mostly quartz, clays and organics. The brine shrimp Artemia gracilis thrives in the lake, and it is a filter feeder that flourishes in spring and summer. It supplies a seasonal rain of faecal pellets. This implies aragonite is a seasonal precipitate within the upper part of the meromictic water column. Rippled sand lamina alternate with laminar mud in the deeper water deposits of Great Salt Lake, with many deeper beds disrupted by the growth of now-dissolved evaporites.

Solar evaporation ponds at the edges of the lake produce salts and brine (see Quaternary potash). Minerals extracted from the lake include: sodium chloride (common salt), used in water softeners, salt lick blocks for livestock, and to melt ice on local roadways (food-grade salt is not produced from the lake, as it would require costly processing to ensure consistent purity); potassium sulfate, used as a commercial fertilizer; and magnesium-chloride brine, used in the production of magnesium metal, chlorine gas, and as a dust suppressant. US Magnesium operates a brine extraction plant on the southwest shore of the lake, which produces 14% of the worldwide supply of magnesium, more than any other North American magnesium operation.

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US Magnesium plant on southwest shore of Great Salt Lake
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Halite and Potash ponds on NE margin of Great Salt Lake

Elevated mercury levels

During a survey in the mid-1990s, U.S. Geological Survey and U.S. Fish and Wildlife Service researchers discovered a high level of methyl-mercury in the Great Salt Lake with up to 25 nanograms per litre of water. For comparison, a fish consumption advisory was issued at the Florida Everglades after water there was found to contain 1 nanogram per litre The extremely high methyl-mercury concentrations have been only in the lake's anoxic deep brine layer (monimolimnion) below a depth of 6.1 m (20 ft), but levels are also moderately high up in the water column where there is oxygen to support brine shrimp and brine flies.

The toxic metal shows up throughout the lake's food chain, from brine shrimp to eared grebes and cinnamon teal. Mines that use cyanide leaching and ore roasting was deemed a likely source.

A more detailed study published in 2010 concluded that the primary source of the mercury is likely worldwide industry, rather than local sources. As water levels rise and fall, mercury accumulation does as well. About 16 percent of the mercury comes in from rivers, and 84 percent comes from the atmosphere in a non-toxic, inorganic form. The non-toxic mercury is converted into toxic methyl mercury by halotolerant bacteria, which thrive best in the more saline water of the North Arm.

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