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Why is the Baltic Sea considered the world’s largest area of brackish water?

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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 Baltic Sea is considered the world’s largest area of brackish water primarily because it is nearly landlocked and receives a massive influx of fresh water from its surrounding rivers.12

The sea's unique hydrological characteristics contribute to this status:

Limited Exchange: The Baltic is connected to the North Sea through very shallow outlets, which restricts the inflow of denser, saline ocean water.2
High Freshwater Input: The catchment area that drains into the Baltic is approximately four times larger than the sea itself. Rivers, such as the Vistula and the Oder, provide a constant supply of fresh water.12
Low Evaporation: The region experiences a temperate continental climate with low evaporation rates, preventing the concentration of salts that occurs in more isolated, warmer basins.2

These factors result in low salinity levels throughout the sea, ranging from about 10 to 15 parts per thousand in the west to nearly fresh water at the head of the Gulf of Bothnia.23