Everyone knows that river water is fresh, and sea water is salty.
How does this happen? Let's start with the background.
There is always water in the atmosphere in the form of drops or vapors. There is especially a lot of water in the lower layers of the atmosphere, where rain or snow clouds often collect. This water is fresh, because it appeared in the atmosphere as a result of evaporation from the surfaces of all water bodies located on Earth - oceans, seas, lakes. Water enters the river bed in the form of rain from a rain cloud. Snow falling from a snow cloud melts in spring, turning into fresh water, filling river beds and often causing flooding in many parts of the earth. This is the so-called water cycle in nature - a process that has been going on continuously since the appearance of an atmosphere containing moisture on Earth.
Water flowing along the surface of the earth dissolves various minerals along the way, including salts, which can give the water a salty taste. The channels of rivers flowing to lakes and seas are constantly being replenished with new water from the atmosphere. The water in the rivers does not have time to become salty, because it quickly flows into lakes and seas, not having time to get saturated with salts and other minerals. Therefore, the water in the rivers is never salty.
However, rivers flow not only along the surface of the earth, but also in its depths, in karst cavities in the mountains. Such water moves inside mountain ranges along karst voids for a very long time. Water that has fallen to the surface of the earth high in the mountains does not always come out at the foot and falls into the bed of flat rivers flowing into the sea.
As a rule, such underground water gets into the depths of the earth and there it forms underground lenses filled with the purest … mineral water. The well-known narzans and other mineral waters, which the Caucasus is famous for, come to man no longer in a fresh form, but filled with minerals useful for health, including salts. There is nowhere for this water to evaporate, since it has always remained underground and dissolved all the surrounding rocks over the long years of its history. Of course, such water only initially flows in the form of underground rivers. However, it eventually becomes salty, although it is not marine.