Why is the Dead Sea so salty, and why do people float?

The Dead Sea is salty because water flows in but cannot flow out. The only exit is evaporation, which removes water and leaves salt and minerals behind. The water becomes so dense that a person is held up easily, which is why swimmers float with little effort. [1] [2]

Salt in seas generally comes from rain. Carbon dioxide makes rainwater slightly acidic, and it weathers rocks and releases mineral ions that runoff carries to the water. Sodium and chloride make up over 90% of the ions in seawater, and about 3.5% of seawater by weight is dissolved salt, according to the Natural History Museum. The Dead Sea receives the same kind of input, mainly from the Jordan River. [3] [2]

What sets the Dead Sea apart is that it is a terminal lake. NOAA describes fresh water that can only flow in, not out. The region is arid, with average rainfall of only two to four inches a year, so evaporation is intense. Evaporation takes away pure water and leaves the dissolved material, so the concentration keeps rising. The Natural History Museum describes isolated bodies of water becoming hypersaline in this way. [1] [3]

The sources give somewhat different figures. NOAA says surface water is about five to nine times as salty as the oceans. NASA says the concentration is above 30 percent, nearly ten times the ocean. A salinity activity from the Aquarius mission also says about ten times. These differ partly because salinity varies with depth and season, and partly because the sources round differently. NOAA notes that below about 100 meters the water is saturated at 332 parts per thousand, so extra salt precipitates and piles up on the bottom. [1] [4] [5]

The lake sits in a rift valley with its surface about 430 meters below sea level, the lowest dry land on Earth. NASA reports that water levels have been falling about 1.2 meters per year, largely because water is diverted from the Jordan River. As the level drops, the lake grows saltier, and the salt layer on the lakebed has thickened by about 10 centimeters a year for four decades. [4] [1]

A floating object displaces water that weighs as much as the object. A person is less dense than ordinary water, so people can float in it, but salt water is denser still. Density rises with salinity and falls with temperature. In the Dead Sea's dense water, the upward push is greater, so a swimmer only needs to recline. NOAA adds that the buoyancy makes swimming in it hard. [6] [5] [1]

The effect is not unique. The Natural History Museum notes that people also float easily in other hypersaline waters, such as Senegal's Lake Retba. The same salinity makes life difficult: the Dead Sea supports little besides microbes. [3] [5]