The water in rivers is generally fresh. This is because its main source is rain and melting snow. This water is distilled because it is purified through the process of evaporation and condensation.
As for the seas and oceans, the water in them is usually salty. This is explained by a long evolutionary process in which water-soluble salts and minerals accumulate in reservoirs over the years. The main source of salts are rivers. Additionally, salt concentration in oceans and seas is heterogeneous.
The main mechanism that keeps seawater salty is evaporation. During this process, clean water mixes with the atmosphere, leaving behind minerals and salts.
Source: Ferra
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