Rivers flow into the sea constantly, yet rivers are fresh and the sea is salty. The secret is where the water goes next.
In this article
Rain dissolves rock
Rain is very slightly acidic because it absorbs carbon dioxide from the air. As it soaks into soil and runs over rocks, it slowly dissolves minerals, releasing ions such as sodium, calcium and potassium. Underwater volcanoes and vents add others, including chloride.
Rivers carry it to the sea
River water does contain dissolved minerals — just in very low concentrations, so it tastes fresh. Rivers keep delivering these small amounts to the oceans.
Evaporation leaves salt behind
The Sun evaporates water from the ocean surface. Water leaves as vapour (which later falls as fresh rain); the dissolved salts stay. Over hundreds of millions of years, the oceans have built up to about 3.5% salt — roughly 35 grams in every litre, mostly sodium chloride.
Why rivers stay fresh
Rivers are constantly refilled by fresh rainwater and keep moving, so salts never have time to concentrate. Lakes with no outlet behave like little oceans — that is why the Dead Sea and Sambhar Lake in Rajasthan are salty.
Is the sea getting saltier?
Not noticeably. Salt also leaves the ocean — trapped in sediments on the sea floor — so overall saltiness has been roughly stable for a very long time.
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