The question I typed was “is the caspian sea salty?”, which I assumed had a one word answer.

It has a one sentence answer. The Caspian Sea has a salinity of about 1.2%, or 12 grams per litre, which is roughly a third of the salinity of average seawater. So yes, it is salty. It is brackish rather than briny. You would not want to drink it, and you would not mistake it for the Pacific either.

But why is it salty at all, and why is it not saltier?

Why it is salty at all

Rivers are fresh, and the Caspian is fed by rivers. More than 130 of them, in fact, with the Volga alone supplying around 80% of the inflow, plus the Ural, the Kura, the Terek and others. If you pour fresh water into a basin, you might reasonably expect fresh water to come out.

It does not, because nothing comes out. The Caspian has no outlet to the ocean. It is an endorheic basin, a closed system whose only real exit is evaporation.

Fresh water is not perfectly fresh. Every river carries a small load of dissolved minerals, scraped off the rocks and soils of its catchment, which for the Caspian covers 3.6 million square kilometres. When that water reaches the sea, the water leaves as vapour and the minerals stay behind. Repeat for a few million years and you have salt. It is the same mechanism that makes the Great Salt Lake and the Dead Sea what they are.

Why it is not saltier

The Dead Sea is about ten times as salty as the ocean. The Caspian is a third as salty. Same basic mechanism, wildly different result.

The difference is the size of the tap relative to the size of the puddle. The Volga is an enormous river pouring continuously into a basin that, while huge at around 371,000 square kilometres, is not huge enough to evaporate that inflow away without diluting itself. The Caspian sits at an equilibrium between the salt the rivers bring in and the fresh water they bring in, and that equilibrium happens to land at roughly a third of ocean strength.

You can watch the balance shift across the map. In the north, where the Volga arrives, the water is nearly fresh. It grows more brackish as you move south, and it is at its saltiest along the Iranian shore, where the surrounding catchment contributes very little flow.

So “how salty is the Caspian Sea” has no single answer. It depends entirely where you put the bucket.

The corner that is saltier than the ocean

Off the eastern edge of the Caspian, in Turkmenistan, there is a shallow lagoon called Garabogazköl, connected to the main body by a narrow strait.

Water flows in. Nothing flows out. It sits in a hot, dry basin and evaporates.

Garabogazköl routinely exceeds oceanic salinity by a factor of ten. It is the Caspian’s own Dead Sea, hanging off its side, and it exists because it is a closed basin attached to a closed basin. In the 1980s the Soviet Union dammed the strait to stop what it saw as a waste of Caspian water. The lagoon dried up. The dam was later removed and it refilled.

Sea or lake?

Both, depending on who is asking and why.

By any physical definition it is a lake: an inland body of water with no connection to the ocean. It is usually described as the largest lake in the world, with a surface area comparable to Japan.

The reason anyone argues is that the answer is worth billions of dollars. Under international law, a “sea” is divided between coastal states under maritime rules, while a “lake” is shared under a different regime entirely. Five countries border the Caspian and there is a great deal of oil beneath it. The word “sea” in its name is doing a lot of quiet legal work.

Its salinity is part of that argument, which is a strange fate for a number. At 1.2% it is too salty to be freshwater and far too fresh to be oceanic, which means it can be cited comfortably by either side.

The short version

The Caspian Sea is salty because it has no outlet and its rivers deliver dissolved minerals that stay behind when the water evaporates. It is not very salty because the Volga keeps pouring fresh water in faster than the sun can concentrate it. It ranges from nearly fresh in the north to distinctly brackish in the south, averaging about a third of ocean salinity. And in one lagoon on its eastern shore, the same process runs to completion and produces brine ten times saltier than the sea it came from.

Not bad for a question I expected to answer with “yes”.

Sources

  • Caspian Sea. Wikipedia. Salinity of approximately 1.2% (12 g/L), about a third that of average seawater; over 130 rivers of inflow, the Volga the largest; surface area 371,000 km².
  • Garabogazköl. Wikipedia.