Most lakes on Earth hold freshwater, but a surprisingly large share do not. Saline lakes occupy close to half the total volume of the world’s inland surface waters, a fact that rarely makes it into geography textbooks.1PubMed. Salt to conserve: a review on the ecology and preservation of hypersaline ecosystems Whether a lake ends up fresh or salty depends on something deceptively simple: whether the water has somewhere to go. That single factor creates a spectrum that runs from near-distilled purity under Antarctic ice sheets to brines so concentrated they make the ocean look dilute.
Why Most Lakes Stay Fresh
A typical lake sits in an open basin. Rivers and streams flow in, and at least one outlet drains toward the sea. Because water keeps moving through, dissolved minerals never build up to high concentrations. The lake refreshes itself, and salinity stays low. The Great Lakes, Lake Baikal, and the vast majority of lakes you have visited all work this way. Freshwater is conventionally defined as having dissolved salts below about 0.5 grams per liter (for reference, seawater averages around 35 grams per liter). Most open-basin lakes fall well below that threshold.
Rain and snowmelt do carry trace minerals into freshwater lakes, picked up from soil and rock along the way. But in an open system these minerals pass through quickly. The residence time of water in many freshwater lakes is measured in years or decades, which is long enough for ecosystems to thrive but short enough that salts never pile up.
How Salt Lakes Form
Salt lakes arise when a basin has no outlet to the ocean. Hydrologists call these endorheic or closed basins, and they cover roughly 18% of the Earth’s land surface.2PubMed Central. Recent global decline in endorheic basin water storages Water flows in but the only way out is evaporation. When water evaporates, it leaves its dissolved minerals behind. Over centuries and millennia, those minerals accumulate. The lake grows saltier with every evaporative cycle, sometimes to extreme concentrations.
The Great Salt Lake in Utah, the Dead Sea, and Lake Urmia in Iran all formed this way. Climate matters too: most of the world’s saline lakes sit in arid or semiarid regions where evaporation outpaces inflow, accelerating the mineral buildup. But aridity is not an absolute requirement. Closed basins also exist at high latitudes and in volcanic terrain, creating salt lakes in places you might not expect.
The Salinity Spectrum
Calling a lake either “freshwater” or “saltwater” is a bit like calling food either “bland” or “spicy.” There is a wide range in between. Limnologists generally recognize several categories: fresh (below about 0.5 g/L), brackish (roughly 0.5 to 30 g/L), saline (30 to 50 g/L), and hypersaline (above 50 g/L). Many lakes hover in the brackish zone and shift categories depending on seasonal rainfall, upstream water diversions, or long-term climate trends.
Not all salt lakes even taste the same, chemically speaking. The Great Salt Lake is dominated by sodium chloride, much like the ocean. But a whole class of saline lakes known as soda lakes are dominated by sodium carbonate and bicarbonate, which pushes their pH above 9, making them strongly alkaline.3PubMed Central. A review of the defining chemical properties of soda lakes and pans: An assessment on a large geographic scale of Eurasian inland saline surface waters In East Africa, soda lakes form in volcanic terrain along the Rift Valley, where the surrounding rock produces unusually alkaline runoff. Evaporation then concentrates these alkaline ions further.4Journal of Geochemical Exploration. Geochemistry of African Soda Lakes Lakes Natron and Magadi are famous examples, with caustic waters that can burn unprotected skin.
At the extreme end sits Don Juan Pond in Antarctica’s McMurdo Dry Valleys. It is generally regarded as the most saline natural body of water on the planet, with salts making up about 40% of its mass. Its chemistry is unlike any other lake: roughly 90% of the dissolved salt is calcium chloride rather than the sodium chloride found in most saline systems.5Nature Publishing Group (Scientific Reports). Don Juan Pond, Antarctica: Near-surface CaCl2-brine feeding Earth’s most saline lake and implications for Mars The water is so concentrated that it stays liquid well below the normal freezing point.
When a Single Lake Is Both Fresh and Salty
Some lakes do not fit neatly on the spectrum because their salinity changes with depth. In a meromictic lake, a layer of denser, saltier water sits permanently at the bottom while a fresher layer floats on top. The two layers do not mix the way they do in an ordinary lake, where wind and temperature changes churn the water column seasonally. This permanent stratification can persist for decades or longer.
A subtropical urban lake studied over the past decade, for example, showed bottom-layer salinity consistently about 2.5 practical salinity units higher than surface water hovering around 2 PSU.6PubMed Central. Changes in hydrodynamics and water quality of a subtropical meromictic urban lake That difference may sound small, but it is enough to create a chemical and biological boundary within the same body of water. In the Vestfold Hills of East Antarctica, meromictic lakes form when seasonal meltwater caps a denser saline layer underneath, and the freshwater lens further suppresses mixing by shielding the deeper water from wind.7Antarctic Science. The meromictic lakes and stratified marine basins of the Vestfold Hills, East Antarctica In these systems, asking whether the lake is fresh or salty depends on where in the water column you sample.
The Caspian Sea Problem
Large saline water bodies sometimes blur the line between “lake” and “sea.” The Caspian Sea is the best-known case. It has no connection to the ocean, which technically makes it a lake. But it stretches across five countries, holds roughly a third of the salinity of ocean water, and supports fisheries more reminiscent of a marine environment than a pond. Biogeographic studies have attempted to classify it using physical variables like sea surface temperature, bathymetry, and seasonal salinity variation, all of which vary dramatically across its north-south extent.8Biogeosciences. Biogeographic classification of the Caspian Sea The shallow northern basin receives heavy freshwater inflow from the Volga River and can be nearly fresh, while the deeper southern basin is distinctly saline. Calling it simply a “lake” or a “sea” misses how much internal variation one water body can contain.
Coastal lagoons add another wrinkle. These shallow water bodies sit at the boundary between land and ocean, and their salinity swings wildly depending on tides, river flow, and wind. Strong storms or heavy rainfall can temporarily push a lagoon toward freshwater, while calm dry spells let seawater intrude and raise salinity.9Journal of Marine Science and Engineering. Impact of Extreme Wind and Freshwater Runoff on the Salinity Patterns of a Mesotidal Coastal Lagoon Whether you call these “lakes” is partly a naming convention, but they illustrate how fluid the fresh-to-salt boundary can be.
Life at the Salty End
Freshwater lakes support the fish, plants, and invertebrates most people picture when they think of lake life. Salt lakes are a radically different biological world. As salinity rises, species diversity drops steeply, but the organisms that remain are remarkable specialists.
Brine shrimp (genus Artemia) are the poster species of hypersaline lakes. They possess an osmoregulation system efficient enough to handle salt concentrations up to ten times that of ordinary seawater.10PubMed Central. The brine shrimp artemia: adapted to critical life conditions At the highest salinities, though, even brine shrimp drop out. The last organisms standing are typically haloarchaea, single-celled microbes from the domain Archaea that can grow in saturated sodium chloride solutions.11PubMed Central. Life at low water activity These haloarchaea use fundamentally different strategies from freshwater organisms to cope with the osmotic stress of living in brine, and they dominate the microbial communities of the saltiest environments on the planet.
The biology of salt lakes is not just of academic interest. Brine shrimp cysts harvested from the Great Salt Lake supply a global aquaculture industry, and the microbial enzymes found in halophilic organisms have industrial applications in biotechnology. Meanwhile, freshwater diversions that reduce inflow to salt lakes can crash brine shrimp populations, rippling up to migratory birds that depend on them as a food source.
Freshwater Lakes Turning Salty
Salinity is not fixed. Human activity is pushing many formerly fresh lakes toward brackish conditions, and the primary culprit in cold-climate cities is road salt. In the United States and Canada, millions of tons of sodium chloride are spread on roads each winter. Runoff carries that salt into storm drains, streams, and ultimately into lakes. Research on urban lakes in the Twin Cities metropolitan area of Minnesota found sodium and chloride concentrations 10 and 25 times higher, respectively, than in comparable non-urban lakes in the same region.12Science of The Total Environment. Increase of urban lake salinity by road deicing salt Historical records from 38 lakes in that region showed rising salinity from 1984 to 2005 that tracked closely with the amount of rock salt purchased by the state.
Road deicing salt does improve safety, cutting vehicular accident rates by more than 78% in treated regions.13Frontiers in Ecology and the Environment. Road salts, human safety, and the rising salinity of our fresh waters But the ecological trade-off is real. Chloride is conservative in freshwater systems, meaning it does not break down or get absorbed by sediment. Once it enters a lake, it stays until flushed out by throughflow, and small urban lakes with lots of paved watershed and little flushing capacity are most vulnerable. Some of these lakes are approaching chloride levels that are harmful to freshwater invertebrates and fish. The trend is not limited to Minnesota: similar patterns have been documented across the northern United States and southern Canada wherever winter road-salt use is heavy.
What Happens When Salt Lakes Shrink
While some freshwater lakes are gaining salt, many salt lakes are losing water entirely. Diversion of inflowing rivers for agriculture and urban use has devastated the Aral Sea, Lake Urmia, and the Great Salt Lake over recent decades. When a salt lake shrinks, it exposes vast stretches of lakebed encrusted with salts and fine sediment. Wind picks up this material and carries it as saline dust storms.
Around Lake Urmia in Iran, researchers have characterized the dust blowing off the drying lakebed and found it carries high concentrations of fine-grained saline and alkaline particles, distinct from ordinary desert dust in its chemical composition.14PubMed Central. Characterization of saline dust emission resulted from Urmia Lake drying The Great Salt Lake faces a parallel problem. Research on dust from its exposed sediments showed that inhaled particles triggered lung inflammation in mice, with elevated markers of immune response and mucus production.15PubMed Central. Pro-inflammatory effects of inhaled Great Salt Lake dust particles The dust contained a cocktail of metals, salts, and bacteria. For communities downwind, shrinking salt lakes are not just an environmental abstraction but a direct respiratory health concern.
Lithium and Other Economic Resources in Salt-Lake Brines
Salt lakes are not just ecological curiosities. Their concentrated brines contain economically valuable minerals, and lithium is the headliner. Global lithium reserves are estimated at roughly 32 to 52 million tons, and about 59% of those reserves sit in salt-lake brines.16ScienceDirect (Elsevier). Research progress on lithium extraction from salt-lake brine The Salar de Atacama in Chile, the salt flats of Bolivia, and several basins on the Tibetan Plateau are major sources. Extracting lithium from brine is considerably cheaper than mining it from hard rock, with cost reductions estimated at 30% to 50%. As global demand for lithium-ion batteries grows, salt lakes and their brines are moving from geological footnotes to strategic resources, sparking political and environmental debates in the regions where these deposits exist.
Beyond lithium, salt-lake brines have long been harvested for sodium chloride, potash, magnesium, and bromine. Solar evaporation ponds at the Great Salt Lake and the Dead Sea are industrial-scale operations that have been running for decades. The economics depend on the specific chemistry of each lake’s brine, which is why the chemical diversity discussed earlier matters: a sodium chloride lake yields different products than a soda lake or a calcium chloride pond.
Why Some Salt Lakes Are Pink
If you have seen aerial photographs of strikingly pink lakes in Western Australia, Senegal, or parts of Spain, the color comes from biology. Halophilic (salt-loving) bacteria and algae produce carotenoid pigments as part of their cellular machinery, and at the densities these organisms reach in salty, sunlit water, those pigments tint the entire lake pink or red.17Journal of Arid Environments. The pink colour of lakes, with an example from Australia The color is most intense in sodium-chloride-rich brines and tends to deepen when salinity and temperature are high, conditions that favor dense blooms of halophilic microbes. Freshwater lakes do not turn pink because these organisms cannot compete in low-salt conditions. The phenomenon is a visible signature of extreme salinity and the specialized communities that thrive in it.
Salt Lakes as Climate Archives
Because salinity in a closed-basin lake tracks the balance between evaporation and precipitation, ancient lake sediments can serve as long-term climate records. Fossil algae preserved in lakebed cores reflect past salinity conditions, and by extension, past drought or wet periods. On the Great Plains of North America and in Africa, reconstructions based on fossil diatom assemblages have uncovered prolonged droughts over the past few millennia that far exceeded anything captured in modern instrumental records.18Journal of Phycology. Tracking long-term changes in climate using algal indicators in lake sediments Similar work on a postglacial saline lake in Canada’s Yukon has used diatom-based salinity estimates to map Holocene climate shifts in the Subarctic.19The Holocene. Multi-proxy Holocene palaeoclimatic record from a saline lake in the Canadian Subarctic
This line of research has practical implications. If droughts far worse than any in recorded history have occurred repeatedly in the past few thousand years, current water-management plans may be based on an artificially mild understanding of what is normal. Salt lakes, by faithfully recording evaporation-precipitation balance in their sediment chemistry, offer a longer and more honest baseline.
Salt Lakes and the Search for Life on Mars
Astrobiologists pay close attention to Earth’s saline lakes because Mars once had them too. Orbital and rover data have identified dried lake basins on the Martian surface that contain sulfate and chloride mineral deposits consistent with evaporative concentration, the same process that creates saline lakes here. Researchers studying a hypersaline sulfate lake on Earth have argued that analogous sulfate-rich closed-basin paleolakes on Mars would be excellent places to search for preserved organic material, concentrated by evaporation and entombed in sulfate minerals.20Frontiers in Microbiology. Microbial Diversity in a Hypersaline Sulfate Lake: A Terrestrial Analog of Ancient Mars Acidic hypersaline lakes in Western Australia have been described as among the best modern terrestrial analogs for past Martian environments.21PubMed. Molecular analysis of the microbial communities of Mars analog lakes in Western Australia
The logic is straightforward: if life can survive in the harshest brine environments on Earth, then similar environments on ancient Mars, which was warmer and wetter billions of years ago, might have supported microbial life as well. And because evaporative minerals are good at trapping and preserving organic molecules, dried salt-lake beds are high-priority targets for future Mars sample-return missions.
Lakes Hidden Under Ice Sheets
Not all lakes sit on the surface. More than 400 subglacial lakes have been identified beneath the Antarctic ice sheet, sealed under kilometers of ice and completely isolated from the atmosphere. Despite the crushing pressure and eternal darkness, these environments are liquid because of geothermal heat from below and the insulating weight of the ice above. Where radio-echo sounding has penetrated to the lake floors, the data indicate that the water is very pure and fresh.22Earth-Science Reviews. Antarctic subglacial lakes Because electromagnetic signals are absorbed more strongly by salty water, the clarity of the reflections tells researchers the salinity is low. Lake Vostok, the largest known subglacial lake, is estimated to hold a volume of water comparable to Lake Ontario, all of it fresh and potentially harboring microbial ecosystems that have been cut off from the rest of the biosphere for millions of years.