Why Is the Dead Sea So Salty? The Science Explained

The Dead Sea is so salty because it is a lake that can only take in and never give out. Water arrives carrying dissolved minerals from the Jordan River and mineral-rich springs, but there is no outlet to the ocean — the only way out is evaporation, and evaporation leaves every gram of salt behind. Repeat that one-way exchange for thousands of years under a desert sun, and you get water holding roughly 340 to 350 grams of salt in every litre: about ten times the concentration of seawater.

A Lake With No Exit

The Dead Sea lies on the floor of the Jordan Rift Valley, with Jordan on its eastern shore and Israel and Palestine on its western one. In 2022 its surface stood about 436 metres below sea level, and it has kept sinking since, so its shores are the lowest exposed ground on Earth. Geographers call it an endorheic, or terminal, basin: the rim of the basin stands higher than the lake's surface, so water physically cannot drain away to the sea. Its main inflow is the Jordan River, which gathers its headwaters from springs around Mount Hermon — the Hasbani, the Banias, and the Dan — crosses the Sea of Galilee, then follows the rift south to the lake's northern end, picking up wadis and saline springs along the way. Every litre carries small traces of minerals dissolved from rock and soil, and every trace ends up here with nowhere further to go. Rivers are the conveyor belts of dissolved minerals everywhere — a role the Amazon plays on a far grander scale — but only in closed basins like this one does nothing ever leave. It is a predicament the disappearing Aral Sea shares, with far grimmer consequences.

Evaporation's Patient Arithmetic

The climate does the concentrating. The Dead Sea sits in a hot, dry desert where summers are scorching and rainfall is scarce, and the sun lifts about a metre of water off its surface every year; two years of direct measurements published in 2018 put the figure at roughly 1.1 metres. That is far more than falls as rain. The loop is simple: inflow arrives, water leaves as vapour, salts stay. Over thousands of years, trace minerals have built up to the 340-odd grams per litre the lake holds today. And people are now speeding the process up. The lower Jordan once delivered an estimated 1.3 billion cubic metres of water a year; after decades of diversions for farms and towns upstream, a 2013 study by a UN commission and a German federal institute put its flow into the Dead Sea at just 20 to 200 million cubic metres. Israel's Environmental Protection Ministry estimates that the lake is now dropping by about 1.1 metres a year. Less fresh water coming in means each year's evaporation concentrates what remains a little further — the same desert heat engine that makes the Sahara a desert is, in miniature, manufacturing brine here.

Not the Salt on Your Table

What makes the Dead Sea remarkable is not just how much salt it holds, but what kind. In seawater, sodium and chloride make up more than 90 percent of the dissolved ions, according to the US Geological Survey; chemically, the ocean is a weak solution of table salt. The Dead Sea leans the other way. A 2024 study in Frontiers in Microbiomes described its brine as about 2 moles of magnesium, 1.5 moles of sodium and 0.5 moles of calcium per litre, balanced by roughly 7 moles of chloride, which makes magnesium chloride, not sodium chloride, the dominant salt. Part of the reason is that ordinary salt keeps dropping out. Halite, the crystalline form of sodium chloride, has been settling onto the lake floor for thousands of years, and so much sodium has left the water that the magnesium and calcium salts, which stay dissolved far longer, now dominate what remains. The result is a heavy liquid. Israel's Oceanographic and Limnological Research institute gives its density as about 1.24 grams per cubic centimetre, roughly a quarter more than fresh water. The brine is also rich in bromine: ICL, which extracts minerals on the Israeli shore, reports 4,500 to 5,000 parts per million in the lake water, far more than the trace amounts in seawater. Bromine, potash and magnesium are recovered by pumping brine into solar evaporation ponds at the shallow southern end, which makes the Dead Sea as much a chemical works as a wonder. The black shoreline mud and mineral salts feed a spa and cosmetics trade, and Danish psoriasis patients, for example, are treated at Ein Gedi, on the western shore, with four-week courses of sun exposure and bathing.

Even the lake's oddest product has a long history. Blocks of almost pure asphalt, the largest recorded at up to 100 tonnes, occasionally rise from the lake bed and float on the surface; the first-century historian Josephus compared the floating lumps to headless bulls. Egyptian embalmers put that bitumen to use. In a 2016 chemical survey of mummies, bitumen turned up in about half of those from the New Kingdom to the Late Period and in 87 percent of Ptolemaic and Roman ones, and most of it traced back to the Dead Sea.

Why You Float Like a Cork

That density is what makes swimming in the Dead Sea feel like a magic trick. Archimedes' principle says a submerged body experiences an upward force equal to the weight of the water it displaces. Because displaced Dead Sea water weighs so much more than freshwater, far less of your body needs to sink before the upward force balances your weight — so you bob at the surface with barely any effort, the classic photograph of someone reading a newspaper on their back. Utah's Great Salt Lake, another lake with no outlet, gives a milder version of the same lift: Utah Geological Survey records show its southern bay averaging about 12 percent salt over nearly 50 years and its northern bay about 25 percent, against roughly 3.5 percent for the ocean. But floating is not the same as safe. A person can still roll face-down, panic, or struggle to control their position, and the water stings eyes, mouths, and any open cut fiercely. Visitors are told not to dive, not to swallow the water, and to stay at designated bathing areas. As for the name: at roughly ten times ocean salinity, the Dead Sea is far too harsh for fish, plants, and typical marine animals to survive — which is exactly how it got its name. Even so, it is not completely lifeless. After unusually rainy winters dilute the surface, blooms of a salt-loving alga and of salt-loving microbes have appeared in the lake. The Dead Sea is a slow-motion chemistry experiment — a basin that collects and keeps, concentrating the desert's mineral harvest into water dense enough to sit on. It is also a gauge: its steadily falling level measures every litre diverted upstream, and its strange mineral recipe is a reminder that "salt" is never just one thing. Salt, in the end, is just water's memory of everything it carried.