Salt Questions · Science, chemistry & environment
Why is the ocean salty but rivers are fresh water?
Short answer
Rain is slightly acidic and slowly dissolves minerals from rocks. Rivers carry these dissolved salts to the sea, but their water is constantly replaced, so they stay fresh. In the ocean, water leaves mainly by evaporation, which leaves the salt behind. Over hundreds of millions of years, the sea has reached about 3.5% salt.
Dissolved salts, grams per litre
River figure is a typical order of magnitude; it varies by river.
Where the salt comes from
The U.S. Geological Survey explains that rainwater absorbs carbon dioxide and becomes slightly acidic. As it runs over and through rocks, it dissolves tiny amounts of minerals and carries them to rivers as ions. Hydrothermal vents and volcanoes on the sea floor add more.
Why rivers stay fresh
River water contains salts too, but at very low concentrations, and it is constantly flushed downstream and replaced by new rain. Rivers are a conveyor belt for salt, not a store.
Why the ocean stays salty
Water leaves the ocean by evaporation, which takes only pure water. The salts stay. Over geological time the oceans reached a rough balance, with salt added by rivers and vents matched by salt removed into sea-floor sediments. The result is about 35 grams of salt in every litre of seawater.
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Cite this page
For school projects and reports. Teachers often prefer the original sources too: see the list below.
The Salt Almanac. (2026, November 8). Why is the ocean salty but rivers are fresh water. https://saltalmanac.com/answers/why-is-the-ocean-salty-but-rivers-are-fresh-water
“Why is the ocean salty but rivers are fresh water.” The Salt Almanac, 8 Nov. 2026, saltalmanac.com/answers/why-is-the-ocean-salty-but-rivers-are-fresh-water. Accessed [today's date].
“Why is the ocean salty but rivers are fresh water.” The Salt Almanac. Last modified November 8, 2026. https://saltalmanac.com/answers/why-is-the-ocean-salty-but-rivers-are-fresh-water.
Original sources
- U.S. Geological Survey. Why is the ocean salty? https://www.usgs.gov/faqs/why-ocean-salty
- Wikipedia. Seawater. https://en.wikipedia.org/wiki/Seawater