Finland, Gulf of

Finland, Gulf of, arm of the Baltic Sea, north central Europe, extending east for about 400 km (250 mi) between Finland on the north and Estonia and Russia on the south. At the extreme eastern end of the gulf is the Karelian Isthmus, separating it from Lake Ladoga. The gulf varies in width from 19 to more than 130 km (12 to 80 mi), its narrowest point occurring at the eastern end. Numerous islands are in the gulf, mainly off the Finnish coast. By means of the Neva River and the New Ladoga Canal, the gulf is linked with Ladoga and Onega lakes to the east, and by means of the Volga-Baltic Waterway and the Volga-Don Canal, the gulf is linked with the Caspian and Black seas. To the south, the Narva River links the Gulf of Finland with Lake Peipus. Sandbars, rocks, and winter ice make navigation difficult. The depth of water in the gulf rarely exceeds 100 m (330 ft). Because of its low salinity, the Gulf is usually frozen from December until March. It is an important shipping route, and its main ports include Helsinki (Finland), Tallinn (Estonia), and Vyborg and St Petersburg (Russia).