If you've ever swum in the sea and come up with your lips stinging and your skin tasting of salt, you've met one of the planet's oldest facts face-first. Sea water is salty. River water, for the most part, is not. And that puzzle has a way of nagging at you once you notice it — because almost every drop of the sea got there by way of a river. So where does all the salt come from, and why does it stay?
Start with the scale. Around ninety-seven percent of the water on Earth's surface is saline, and the oceans and seas hold almost all of it. The dissolved salt adds up to so much that if you could somehow dry the seas out and pile the residue on land, you'd be looking at a layer roughly a hundred and fifty metres deep — a wall of salt tall enough to swallow a forty-storey building. All of it dissolved, invisible, and evenly mixed into water you can swim in.
Where the salt comes from
The water cycle is the delivery system. Rain begins as pure water vapour, but as it falls it brushes against carbon dioxide in the atmosphere and turns very slightly acidic — weak carbonic acid, the same mild bite that gives rainwater its faint tang. That weak acid is a patient sculptor. Running down mountainsides as streams and then rivers, it slowly attacks rock, dissolving minerals and carrying the fragments along. Sodium, chloride, calcium, magnesium, sulphate: the water gathers a little of each, the way a river gathers silt.
None of this makes a river taste salty, because the amounts are tiny — a few grams in every litre at most. But rivers never stop. Pour a thin trickle of salt into a bucket of water and it stays drinkable. Pour that trickle in every second for a hundred million years, without ever emptying the bucket, and you get an ocean.
And the salt isn't a single ingredient. The dissolved load that reaches the sea is dominated by sodium and chloride — the two that together make up ordinary table salt — with smaller helpings of calcium, magnesium, potassium and sulphate. Where a river runs matters too: a stream cutting through soft, soluble rock carries far more dissolved mineral than one tumbling over hard granite, which is one reason some rivers taste noticeably harder or saltier than others.
Why rivers stay fresh and seas don't
Here's the asymmetry that answers the whole question. Rivers are continually flushed. Rain refills them, and the water flows away to the sea, carrying its dissolved salts out with it. A river is a pipe, not a basin — nothing accumulates for long.
The sea is a basin. When the sun lifts water off the ocean surface, it takes up almost pure water vapour and leaves the salt behind. The salt has no easy exit. So every river emptying into the sea is quietly adding to a stock that has been building for billions of years, while the evaporation only ever removes the water and returns it to land as rain, ready to pick up more minerals on the way back.
That doesn't mean the ocean is getting saltier year by year. Over the long run it reaches a rough balance: salt arrives from rivers and from deep-sea hydrothermal vents, and it leaves through sea spray blown inland, through minerals that lock it into rock, and through the slow recycling of ocean crust. The modern ocean sits at about thirty-five grams of dissolved salt per litre — roughly 3.5 percent — and has held close to that for a very long time.
A time when the sea was different
The balance isn't eternal, though. About six million years ago the Mediterranean was cut off from the Atlantic and largely evaporated, leaving behind enormous salt deposits — an event geologists call the Messinian salinity crisis. Drill into the sea floor there today and you find kilometres of salt, the ghost of a sea that dried. It's a reminder that ocean chemistry isn't fixed; it shifts when the planet's plumbing shifts.
So the next time you taste salt on the wind, or wince at sea water, you're tasting rock that rain dissolved, over and over, across an unimaginable stretch of time. The river gave it up. The sea kept it. That's the whole trick — and it has been running for far longer than there have been eyes to notice.