the eastern arm of the Baltic Sea, situated between the USSR to the east and south and Finland to the north. It covers an area of approximately 30,000 sq km. It is 390 km long and 70 km wide at the entrance, reaching a maximum width of 130 km at Narva.

The northern shore of the Gulf of Finland is strongly indented and rocky, with many skerries. The eastern and southern shores are primarily low-lying and are composed of soft rocks; a glint line of bedrock borders the shores and, in places, extends to the gulf itself. The eastern section of the gulf is called Neva Bay and receives the Neva River. Vyborg Bay is located in the north, and Kopor’e, Luga, and Narva bays are located in the south. Depths decrease from 100 m in the east, at the entrance, to 20–30 m west of Kotlin Island; east of the island the maximum depth is 5 m. Neva Bay is 3–4 m deep, except in the Morskoi Canal, which links the port of Leningrad to water deep enough for large ships. The gulf abounds in islands, the largest of which are Kotlin, Moshchnyi, Bol’shoi Tiuters, Malyi Tiuters, Sommars, Gogland, Naissaar, and Osmussaar. There are many shallows, shoals, and bars near the shores; fish is abundant.

In the winter, the average water temperature at the surface is approximately 0°C; in August it ranges from 15°C at the entrance to 17°C and higher in Neva Bay. The temperature at the bottom is 2° or 3°C. An ice cover forms offshore between late November, in the east, and mid-December, in the northwest; it begins to break up in the west in late April and finishes breaking up at the skerries in the first half of May. The salinity at the surface is 3–6 parts per thousand (‰), decreasing to 2‰ or less near Neva Bay. The water level fluctuates sharply, depending on wind and atmospheric pressure; it rises dramatically in Neva Bay (150–410 cm), causing flooding in Leningrad.

The major Soviet ports of Leningrad, Tallinn, and Vyborg are situated on the Gulf of Finland, as are the Finnish ports of Helsinki and Kotka.