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Why the sea is salty

Rivers carry a thin trickle of salt into the sea. Water leaves, but the salt stays.

4 min read Last updated 3 Nov 2026
A cube of salt sitting in a blue wave, with specks of salt dissolving into the water.

The short version

Rain dissolves tiny amounts of salt and other minerals from rocks, and rivers carry them to the sea. Water leaves the sea by evaporation but the salt does not, so it has built up over a very long time. Volcanoes and vents on the sea floor add some too.

I always wondered why rivers taste fresh but the sea they flow into is salty. It is the same water, after all.

Rivers carry a thin trickle of salt, and they have been doing it for millions of years.

Water leaves the sea. The salt stays.

Rain slowly dissolves rock

Rain is slightly acidic, because carbon dioxide from the air dissolves in it. When it falls on rock and soil, it dissolves tiny amounts of minerals. Chemists call these dissolved minerals salts. The one we sprinkle on chips, sodium chloride, is the most common salt in the sea.

Rivers carry the dissolved salts down to the sea. The amount is so small that you cannot taste it. River water holds about 0.01% dissolved salts. Seawater averages about 3.5%, roughly 300 times as much.

How salty is it?

Grams of dissolved salts in every litre of water. The numbers are averages and vary from place to place.

Rivers (world average) 0.1 g per litre
Baltic Sea (main basin) 7 g per litre
The oceans 35 g per litre
The Dead Sea 340 g per litre

Water leaves, salt stays

The Sun evaporates water from the sea surface. Evaporation takes the water and leaves the dissolved salt behind. The water rises, forms clouds, falls as rain and runs down the rivers back to the sea, picking up a little more salt on the way.

You can see the same thing in a pan of salty water left to dry: the water disappears and a crust of salt stays. Repeat that cycle for a very long time and the sea slowly collects the salt.

Rivers have no salt, so the sea's salt must come from somewhere else.

Rivers do carry salt. There is too little in each glass to taste, but over millions of years the trickle adds up.

Why the sea is not getting saltier

If salt keeps arriving, you might expect the sea to keep getting saltier. It does not, because salt is also being removed.

Sea creatures use some of the minerals to build shells and skeletons. Some minerals settle into the mud on the sea floor. Sea spray carries a little salt onto land. And in shallow lagoons the water sometimes dries out completely, leaving thick beds of salt. The salt mines under Cheshire are what is left of a sea that dried up more than 200 million years ago.

Salt coming in and salt going out are close to balance, so the sea has stayed about as salty as it is now for a very long time.

About 166 m

How deep a layer of salt you would get if all the salt in the sea were spread evenly over the land. That is about 500 feet.

Where does the chloride come from?

Salt is sodium and chloride joined together. Rain and rivers supply plenty of the sodium by wearing away rocks, but they supply much less of the chloride.

Most of the chloride has another source: volcanoes, which throw out gases containing chlorine, and hot vents on the sea floor, where sea water seeps into the rock and comes out hot and full of dissolved minerals. Geologists think the early oceans got much of their chloride this way, and rivers have been adding sodium ever since.

Common salt makes up about 85% of the dissolved salts in the sea. The rest is mostly magnesium, sulphate, calcium and potassium. The sea-floor vents also take some of these, such as magnesium, back out of the water, which helps keep the overall saltiness steady.

Sources:

Key takeaways

  • Rain dissolves minerals from rock, and rivers carry them to the sea.
  • Evaporation removes water but leaves the salt behind.
  • Rivers are salty too, but far too weakly to taste.
  • Salt is also removed, so the sea stays about the same.
  • Volcanoes and sea-floor vents supply a lot of the chloride.

Now you know.

Rivers carry a thin trickle of salt to the sea.

The Sun takes the water away. It cannot take the salt.

So the sea is where the salt ends up.

Last reviewed 3 Nov 2026

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