Gulf of Bothnia

The Gulf of Bothnia (Swedish Bottniska viken; Finnish Pohjanlahti) is the northern branch of the Baltic Sea between Sweden and Finland.

The name is related to the Swedish name for this part of the Baltic Sea, Botten[-]havet (bottom sea). Whether this is the original meaning of the name or a folk etymological reinterpretation is uncertain. It is bordered by the counties of Norrbotten, Västerbotten and Österbotten.

As the Gulf of Bothnia is frozen over for several months of the year, it is of secondary importance for shipping. Nevertheless, there are some port cities, such as Luleå and Oulu.

According to the definition of the International Hydrographic Organisation, the Gulf of Bothnia is bounded on the south by a line extending from the Simpnäsklubb lighthouse (59°54'N) in Sweden across the southernmost Åland Islands to Hanko (59°49'N) in Finland. According to this definition, the Åland Islands, the Åland Sea lying between the Swedish mainland and Åland, and the Archipelago Sea lying off the south-west coast of Finland are parts of the Gulf of Bothnia. According to other definitions, the bay only begins north of Åland.

The Gulf of Bothnia consists of two basins separated by the strait Kvarken: The Bothnian Sea in the south and the Bottenwiek in the north. The salinity decreases from south to north. In the Bothnian Sea, the salinity is low enough to speak of brackish water, while the water in the Bottenwiek is practically fresh water. Therefore, freshwater fish such as pike and perch are also found there.

In total, the gulf has a north-south extension of 725 km, the greatest width is 240 km. At the narrowest point in the Kvarken, the Swedish and Finnish mainland are about 80 km apart. The area is 117,000 km², which is about the same as the two largest German states of Bavaria and Lower Saxony combined. The volume of water is about 7,000 km³, which is one third of the entire Baltic Sea or about twice as much as Lake Huron in North America contains.

The average depth is 60 m, and the greatest depth is 301 m in the Åland Sea, which is the second deepest point in the Baltic Sea after the Landsort Deep. If the Åland Sea is not counted as part of the Gulf of Bothnia, the deepest point is 295 m off the coast at Härnösand in the Bothnian Sea. The deepest point in the Bothnian Sea is the (Luleå Deep) at 147 m.