Why Is the Dead Sea So Salty? Evaporation, a Falling Lake and the Lowest Land on Earth

The Dead Sea is a terminal lake: the Jordan river and a few streams and springs flow into it, and nothing flows out, because it sits at the bottom of the Jordan Rift Valley, a trench where the African and Arabian plates are pulling apart, and there is nowhere lower for the water to go. In a hot, dry climate the only exit is evaporation, and evaporation takes the water and leaves the salt behind. Every river carries dissolved minerals from the rocks it crosses, a trace in any glass of it, and a lake that has been receiving rivers and losing only vapour for tens of thousands of years accumulates the trace into a brine. The ocean is salty for the same reason and is much larger and less concentrated; the Dead Sea is about 34 percent dissolved solids, against 3.5 for seawater, and its salt is unusual, richer in magnesium, calcium and bromide than in sodium, which is why it tastes bitter and feels oily.

The rift that holds the lake is part of the same fault system that runs from East Africa through the Red Sea, and along this stretch the two sides are sliding past each other with a pull-apart between them, so that the floor of the valley has dropped several kilometres over millions of years and filled with sediment and salt. The lake's surface is 430 metres below the Mediterranean, its deepest point 300 metres below that, and beneath the floor lie layers of salt hundreds of metres thick from earlier lakes that dried completely. The whole depression was, in the last ice age, a much larger lake, Lake Lisan, whose shorelines are visible as terraces on the valley sides a hundred metres and more above the present water.

The lake has been used for its salt and its bitumen since antiquity, and its shores hold some of the oldest human sites in the region:

• •Jericho, at the northern end, with a settlement dating to about 9000 BC, among the oldest towns known
• •Masada, the fortress on the western shore where the last Jewish rebels against Rome held out until 73 AD
• •The Qumran caves, where the Dead Sea Scrolls lay in jars for two thousand years in air dry enough to keep parchment
• •The potash works at the southern end, on both the Israeli and Jordanian sides, which evaporate the brine in vast ponds for fertiliser and bromine, and which are now the main body of water in the southern basin
• •The resorts, where the dense water, the mineral mud and the thick air, which filters ultraviolet at that altitude, draw people with skin conditions and everyone else

The lake has fallen about 40 metres since the 1960s and is losing more than a metre a year. The Jordan, which once delivered over a billion cubic metres a year, delivers a tenth of that, since Israel, Jordan and Syria divert most of it for farms and cities, and the potash works pump brine from the northern basin to their evaporation ponds. With less coming in and evaporation unchanged, the lake retreats, and as it retreats the fresh groundwater that follows it down dissolves the salt layers beneath the old shore, so that the ground collapses into sinkholes, thousands of them, which have swallowed roads, a campsite and a section of the main highway. The southern basin dried out in the 1970s and exists now only as the industry's ponds.

The remedies all involve bringing water from somewhere else. A canal from the Mediterranean was studied in the 1980s and a pipeline from the Red Sea, agreed by Israel and Jordan in 2013, would have desalinated water for both countries and pumped the brine 200 kilometres into the Dead Sea to stabilise it; the project was cancelled in 2021 amid the politics of the region and doubts about what mixing seawater into the brine would do, since the two waters differ in chemistry and the mixture might turn the lake white with gypsum or red with algae. Restoring the Jordan's flow is the alternative that hydrologists prefer and that the countries that drink from it have not found a way to afford. The lake will not disappear, since as it shrinks its surface falls and its concentration rises until evaporation slows, and models suggest it will stabilise a hundred metres or so lower, smaller and saltier still, the sinkholes marking where the shore used to be.

The Dead Sea is often called the saltiest body of water on Earth, but it is not. Several smaller lakes and ponds are saltier, including Lake Assal in Djibouti and, most extreme of all, Don Juan Pond in Antarctica, a shallow pool so rich in calcium chloride that it rarely freezes even in the Antarctic winter. What makes the Dead Sea unusual is the combination of very high salinity with its size, depth and long history, and the fact that people can walk into it from a hotel beach.

Nor is it completely lifeless. Hardy microbes, including salt-loving archaea and a single-celled alga, live in the brine, and in unusually rainy winters, when fresh water forms a thin, less salty layer on top, the green alga can bloom, followed by salt-loving microbes that briefly tint the water red. Divers exploring the lake floor have found springs of fresher groundwater seeping up through craters, coated with mats of bacteria that can cope with the sudden changes in saltiness. The lake is also changing chemically as it shrinks. The water has become saturated enough that ordinary salt now crystallises out and settles on the bottom, slowly adding a new layer to the thick salt deposits beneath.

You can work out how high you float with Archimedes' principle: a floating body sinks until it displaces its own weight of liquid, so the fraction under the surface equals the body's density divided by the liquid's. A person with air in their lungs has a density close to that of fresh water, about 1 gram per cubic centimetre, which is why most of you sits just below the surface in a swimming pool. Dead Sea brine is about 1.24 grams per cubic centimetre, so only about four fifths of your volume needs to be submerged, and the remaining fifth rides above the water.

That buoyancy changes how you have to move. Your legs rise so readily that lying on your front tends to tip your face towards the water, and a mouthful or noseful of the brine is dangerous: it is intensely irritating, and inhaling even a little can injure the lungs, so lifeguards tell visitors to float on their backs, keep their heads up and avoid splashing. The water stings badly in the eyes and should be rinsed out at once at the beach showers. To stand up again, draw your knees towards your chest and push your feet down slowly rather than rolling over.

The dry climate that makes the Dead Sea so salty also preserved one of the most important archaeological discoveries of the twentieth century. In 1947 a Bedouin shepherd exploring a cave in the cliffs near Qumran, close to the north-western shore, found clay jars containing ancient scrolls. Over the following decade, thousands of fragments were recovered from eleven caves in the area.

The Dead Sea Scrolls include the oldest known manuscripts of books of the Hebrew Bible, about a thousand years older than the copies previously available, along with other religious and community writings from roughly the third century BCE to the first century CE. The extreme dryness of the caves allowed leather and papyrus to survive for two thousand years.

The Dead Sea is ten times saltier than the ocean because it is a terminal lake at the bottom of a rift valley 430 metres below sea level, from which water leaves only by evaporation while the Jordan and its springs bring in dissolved minerals that stay behind, a brine rich in magnesium and bromide dense enough to float a person. It has fallen 40 metres since the 1960s because the Jordan is diverted and the potash works pump its water, the retreat opens sinkholes along the old shore, and a Red Sea pipeline meant to refill it was abandoned in 2021.