Why are Oceans Salty - But Rivers Are Not?

If you’ve ever swallowed a mouthful of ocean water, you know immediately that seawater and fresh water are very different things.

But here’s an interesting question: If much of the water flowing into the ocean comes from freshwater rivers, why is the ocean salty?

The answer begins with rain, rocks and a journey that can take thousands—or even millions—of years.

It Starts With Rain

Rainwater isn’t quite as pure as you might imagine.

As rain falls through the atmosphere, it absorbs small amounts of carbon dioxide. This makes the water slightly acidic—not dangerously so, but just enough to slowly react with rocks and soil.

As rainwater travels across the land and underground, it gradually dissolves tiny amounts of minerals from the rocks it encounters.

Those dissolved minerals contain electrically charged particles called ions, including sodium and chloride—the two ingredients that make up ordinary salt.

Eventually, streams and rivers carry these dissolved minerals toward the ocean.

So Why Aren’t Rivers Salty?

They actually are.

Rivers contain dissolved salts and minerals, but in concentrations so low that we generally don't taste them.

The ocean is different because it acts somewhat like a giant collection basin.

Water continually enters the oceans from rivers and streams. But when ocean water evaporates under the heat of the sun, the water leaves and the salt stays behind.

Think of a pot of salted water simmering on the stove. As the water evaporates, the salt doesn't disappear with the steam. It remains in the pot.

Now imagine that process occurring across the world's oceans for an extraordinarily long time.

Billions of Years in the Making:

Earth's oceans are ancient, and this cycle has been happening for billions of years.

Rivers continually deliver dissolved minerals to the sea. Underwater volcanic activity and hydrothermal vents also contribute dissolved minerals.

At the same time, minerals are constantly being removed from seawater through natural processes. Some are used by marine organisms, some settle onto the ocean floor, and others react with rocks beneath the sea.

The result is a remarkable natural balance.

Today, the average ocean contains about 35 grams of dissolved salts in every kilogram of seawater—roughly 3.5%.

But not every ocean, or even every part of an ocean, has exactly the same salinity.

Areas with lots of rainfall or freshwater flowing in from rivers tend to be less salty. Hot, dry regions with high evaporation can be considerably saltier.

And Where Did All That Salt Come From?

Mostly from something surprisingly ordinary: Rocks.

The mountains, valleys and landscapes around us are very slowly being weathered by water. Tiny quantities of their minerals are carried downstream, eventually reaching the sea.

A single river doesn't deliver much salt on any given day. But give nature millions upon millions of years, and those tiny contributions add up.

It's one of the wonderful things about water.

A raindrop falling on a mountain, a river flowing toward the coast and the salty water of an ocean may seem like completely different things.

In reality, they're all part of the same enormous journey.

Water is always on the move—and it's always carrying a little bit of its journey with it.