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Why is the ocean salty and rivers aren’t? In fact, most of the salt in the sea comes from rivers. But, how can that be? It all starts with ocean water heading out on its journey: warm surface water evaporates and the water vapor rises to condense into clouds that sometimes get carried over land, where they rain down again. On their way down, the water droplets absorb CO2 and become slightly acidic. When they splash onto stones and soil, they wash out sorts of minerals as ions and feed them into rivers. From there, the salts travel to some lake or the ocean, where they can accumulate over time. After animals and plants feast on this mineral buffet, sodium and chloride ions remain as leftovers; you know them as salt. It’s responsible for the salinity of ocean water because the ocean deposits as much salt sediment on the seafloor as rivers feed into it, and the salt level stays at a stable 35 grams per liter on average. Rivers, in turn, do not accumulate salt; they simply deliver the minerals, including the little traces of salt, to the oceans and lakes. What goes in comes out again.