Some basic facts about the Baltic Sea Region (BSR) after various sources (1):

Baltic catchment area:         about 2,100,000 km²

Shoreline length:                  about 8,000 km

Population in catchment:    85 Million people in 14 countries

Baltic Sea surface area:        about 420,000 km²

Average depth:                     55 m

Maximum depth:                  459 m

Water volume:                      22,000 km³

Water residence time:         about 30 years

Water budget: The annual mean precipitation exceeds evaporation by about 40 km³a^-1 and is a net source of fresh water for the Baltic Sea. With 436 km³a^-1, the mean annual river discharge is more than 10 times higher and the most important source of fresh water. This amount is comparable to the total inflow of saline water from the North Sea (2).

The Baltic Sea water budget, pollution and state is largely controlled by riverine inputs. Major rivers are:

• Neva: catchment area: 287,000 km²; water discharge: 67 km³a^-1; population 6.1 million
• Vistula: catchment area: 193,000 km²; water discharge: 42 km³a^-1; population 23.5 million
• Daugava: catchment area: 86,000 km²; water discharge: 21 km³a^-1; population 2.8 million
• Nemunas: catchment area: 92,000 km²; water discharge: 20 km³a^-1; population 4.9 million
• Oder/Odra: catchment area: 117,000 km²; water discharge: 16 km³a^-1; population 16.2 million.

Salinity: The Baltic Sea is one of the largest brackish water areas in the world. Its surface water salinity gradually changes from 15 - 18 psu (1.5 - 1.8%) in the southwest (the Sound between Denmark and Sweden), 7 - 8 psu in the cental Baltic Sea (Baltic Proper) and 0 - 2 psu in the northeast (Bothnian Bay) (1).

Geology: The Baltic Sea is geologically a very young sea. It was formed after the Weichselian Glaciation (about 12,000 years ago) when the Scandinavian ice sheet retreated. Afterwards it has gone through different salinity states from freshwater to marine. The present state, with a connection to the North Sea, only developed 7,500 and 4,000 years before present, during the Littorina transgression (1).