Science

Why the sea is salty, and why rivers are not

Rivers carry salt to the sea continuously. The interesting question is not why the sea is salty but why the rivers feeding it are fresh.

By 5 min read
Shallow salt pans by the sea, with white salt crystallised at the edges.
Sea salt pans. Photograph by Dey.sandip, CC BY 3.0, via Wikimedia Commons.

Key takeaways

  • Rain and rivers weather rock and carry dissolved ions to the sea.
  • Water leaves the ocean by evaporation, which leaves the dissolved salts behind.
  • Rivers stay fresh because their water passes through, taking its dissolved load with it.
  • Lakes with no outlet, like the Dead Sea, do become salty for exactly the same reason the ocean does.
  • Ocean salinity is roughly steady because salt is removed by sediments and reactions as fast as it arrives.

Every river on Earth carries dissolved salts, and every one of them ends in the sea. That is the ordinary answer to why the ocean is salty, and it is correct as far as it goes. It also raises a better question, which is why the rivers doing the carrying are themselves fresh.

Rain is very slightly acidic, and rock dissolves

Rainwater picks up carbon dioxide from the air and becomes mildly acidic. Falling on rock, over long periods, it dissolves it. This is chemical weathering, and it is slow, relentless and everywhere.

What ends up in the water is a mixture of ions: sodium, chloride, calcium, magnesium, sulphate, potassium. Streams collect them, rivers carry them, and they arrive at the coast. The concentration in any given river is low enough that you cannot taste it, which is why we call it fresh water. It is not pure water; it is very dilute salt water.

Rivers are not the only source. Hot water circulating through fresh volcanic rock at mid-ocean ridges exchanges ions with it and returns them to the sea, which contributes a significant share of the total.

Only the water leaves

Here is the whole mechanism. The ocean loses enormous quantities of water to evaporation, and evaporation is selective: water molecules go into the atmosphere, dissolved ions do not.

So the water cycles. It evaporates, forms cloud, falls as rain, weathers more rock, and returns down the rivers with a fresh load of ions. The salt does not cycle with it. It arrives and stays, and over geological time it accumulates.

A river is a different situation entirely. Water enters at one end and leaves at the other, and the dissolved load leaves with it. Nothing concentrates because nothing is retained. That is the answer to why rivers are fresh: not that they carry no salt, but that they do not keep it.

Why salt accumulates in the sea but not in a riverRain weathers rock and rivers carry dissolved ions to the sea. Water leaves the ocean by evaporation, which carries no salt with it, so the water cycles and the salt stays. A river behaves differently: water enters at one end and leaves at the other, taking its dissolved load with it, so nothing concentrates. A lake with no outlet behaves like a small ocean and turns salty, which is why the Dead Sea and the Great Salt Lake are salty while lakes with rivers running out of them are fresh.Only the water leavesTHE SEAevaporation carries water only — no salt goes uprain weathers rockslightly acidic; dissolvessodium, chloride, calciumrivers deliver the ionssalt arrives and staysremoved only slowly, by burial in sediment,reaction with fresh rock and sea sprayThe test: does the water have an exit?River or lake with an outflow: fresh. Sea, or a lake with no outlet like the Dead Sea: salty.Not size, not age, not what the surrounding rock is made of.
A river is not fresh because it carries no salt. It is fresh because it does not keep any — the dissolved load leaves at the far end. Salinity has been broadly stable for a very long time because removal roughly balances input. Diagram by HRITU.

The test case: lakes with no way out

If the argument is right, then any body of water that receives rivers but has no outlet should behave like a small ocean and turn salty. That is exactly what happens.

The Dead Sea takes water from the Jordan and loses it only to evaporation. It is several times saltier than the ocean. The Great Salt Lake in Utah is the same arrangement with the same result. Basins of this kind are called endorheic, meaning they drain inward with no outflow to the sea, and salt lakes are their characteristic feature.

Meanwhile a lake with a river running out of it, such as Lake Geneva or the Great Lakes, stays fresh, because it is a wide slow section of a river rather than a dead end.

The distinction is not size, or age, or what the surrounding rock is made of. It is whether the water has an exit.

Why it has stopped getting saltier

If salt has been accumulating for billions of years, the ocean ought to be a brine by now. It is not: average salinity is around 35 grams of dissolved salt per kilogram of seawater, and it has been broadly stable for a very long time.

The accumulation picture is missing the exits. Salt does leave the ocean, by several routes. Ions are buried in sediments on the sea floor. They react with fresh basalt at the ridges, running the hydrothermal exchange in reverse for some elements. Shallow seas evaporate entirely and leave thick salt deposits behind, which is where rock salt comes from. And sea spray carries salt inland on the wind.

Input and removal are approximately in balance. The ocean is not a bucket filling up; it is a bath with the tap and the drain both running.

Why sodium and chloride win

Rivers deliver a whole mixture of ions, but seawater is dominated by two. The reason is not what arrives most, but what leaves least.

Calcium is removed briskly, because marine organisms take it up to build shells and skeletons which eventually settle as sediment. Silicon is taken by diatoms for the same sort of reason. Sodium and chloride have no comparable biological sink and are highly soluble, so they stay in solution far longer than most of their companions.

What you taste in seawater is therefore not a snapshot of what the rivers brought in. It is what was left after everything with an efficient exit had taken it.

More of the same reasoning

For another case where a familiar property of water turns out to be unusual, see why ice floats. And for what the sea does twice a day, why the tide comes in twice a day.

Sources

  1. Seawater, on composition and salinity
  2. Endorheic basin, on lakes with no outlet
  3. Weathering

Frequently asked questions

Not appreciably, on human timescales. Salt arrives from rivers and from hydrothermal activity at the sea floor, and it is removed by burial in sediments, by reactions with fresh volcanic rock, and by sea spray depositing salt on land. Those processes are roughly in balance, so average salinity has been broadly stable for a very long time.

Because sodium and chloride are the ions least readily removed once they are in the water. Calcium, by contrast, is taken up continuously by marine organisms to build shells and skeletons, and ends up on the sea floor. What remains dissolved over long periods is what does not have an efficient exit, and sodium and chloride are the clearest examples.

Because the balance of inputs differs. The Baltic is nearly enclosed, receives a great deal of river water, and loses relatively little to evaporation, so it is noticeably brackish. The Mediterranean is warm, evaporates strongly, and exchanges water with the Atlantic through a narrow strait, so it is saltier than the open ocean.

HRITU Editorial Desk is the byline for pieces produced in-house by the HRITU editorial team. We write about cooking, travel, science and everyday knowledge, and we explain the reasoning behind what we publish rather than just stating conclusions. Every article is checked against primary or reference sources before it goes out, dated on publication, and updated in the open when we learn something is wrong. Corrections and questions are welcome at info@hritu.com.

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