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Why is the water near the head of the Gulf of Bothnia nearly fresh?

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Map showing the physical features, major cities, and national boundaries around the Baltic Sea, North Sea, and English Channel regions of northern and western Europe. The map highlights coastal countries including the United Kingdom, Norway, Denmark, Germany, the Netherlands, Belgium, France, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Russia, with sea depths indicated and key islands, peninsulas, and underwater trenches labeled. — Baltic Sea The Baltic Sea, the North Sea, and the English Channel.

The water at the head of the Gulf of Bothnia is nearly fresh primarily because it receives a large volume of river discharge.12

Several factors contribute to this low salinity:

High River Inflow: The gulf is fed by numerous rivers, such as the Ångerman, Ume, Lule, Torne, Kemi, and Oulu, which continuously dilute the seawater.1
Limited Water Exchange: The gulf is relatively isolated from the rest of the Baltic Sea due to its narrow mouth and the presence of the Åland Islands, which restricts the influx of denser, saltier water.13
Seasonal Effects: The salinity levels drop even further during the spring when snow melt significantly increases the volume of fresh water entering the gulf.23

Because of this low salinity and the shallow nature of the coastal waters, the head of the gulf frequently accumulates pack ice during the winter months.2