Rivers carry small amounts of dissolved minerals from weathered rocks into the ocean. Seafloor processes also exchange dissolved substances with seawater. When water evaporates, most dissolved salts stay behind. Fresh river water is not chemically pure water.
How rocks contribute salt to the sea
Rainwater interacts with rocks and soils. Weathering releases dissolved ions, which runoff carries into streams and rivers. The US Geological Survey describes this route from land to ocean. [1] You do not need to see grains of salt in a river for it to transport dissolved material.
“Fresh” describes water with a relatively low salt concentration. It does not mean that every substance except H₂O has been removed. This resolves the apparent contradiction in the title: rivers can be fresh and still deliver salts.
Evaporation moves water, not the whole mixture
During evaporation, water enters the atmosphere as vapour while most dissolved salts remain in the liquid. Salt-making ponds use this principle to concentrate saline water. [3]
Separate two questions: where the salt comes from, and what changes its concentration. Rock weathering helps answer the first; adding or removing water helps answer the second. Evaporation does not manufacture the salt.
The seafloor also participates
Water circulating through hot seafloor rock undergoes chemical reactions. Some dissolved constituents are removed and others enter the water. Underwater volcanic activity can also contribute minerals. Marine organisms and other processes remove some ions from seawater. The ocean is therefore an exchanging system, not a container that only receives salt. [2]
What does 35 parts per thousand mean?
NOAA gives about 35 parts per thousand as an average ocean salinity, equivalent to roughly 3.5% by mass. [2] As a worked example, 1,000 grams of seawater at that salinity contains about 35 grams of dissolved salts and 965 grams of water.
That is a mass calculation, not a recipe for adding 35 grams of table salt to one litre of water. Seawater contains a mixture of ions, and adding salt to water changes the total mass of the mixture.
Salinity varies between places as evaporation, precipitation and freshwater inputs differ. A global average is not a measurement of your local beach. Likewise, the colour of the ocean cannot be used as a direct salinity reading.
Sources and evidence
- US Geological Survey — Why is the ocean salty? ↗
Weathering, dissolved ions and river transport.
- NOAA Ocean Service — Why is the ocean salty? ↗
Seafloor exchange, ion removal, salinity and its variation.
- US Geological Survey — Evaporation and the water cycle ↗
Evaporation and salt production in evaporation ponds.
Sources consulted: 9 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: First publication.