The Vistula Lagoon (Polish: Zalew Wiślany or Zatoka Fryska in 1910 ; Russian: Калининградский залив or Kaliningradskiy Zaliv; German: Frisches Haff; Lithuanian: Aistmarės) is a fresh water lagoon on the Baltic Sea separated from Gdańsk Bay by the Vistula Spit. It is sometimes known as the Vistula Bay or Vistula Gulf.
Localities on the lagoon include Kaliningrad, Baltiysk, and Primorsk in Russia's Kaliningrad Oblast and Elbląg, Tolkmicko, Frombork, Krynica Morska in Poland. The Polish port of Elbląg used to see a substantial amount of trading traffic on the lagoon, but that has declined owing to the current border situation. Kaliningrad and Baltiysk are currently major seaports on the lagoon.
Digging a canal to connect the lagoon with the Baltic Sea is in consideration as a major EU-supported project. The canal, 1km in length, would re-activate the Elblag port. It would also free its dependence on Russia, which time and again revokes the right of passage for Polish ships through Strait of Baltiysk as a form of pressure on Polish authorities. It would cost an estimated PLN 80 million. However, major ecological considerations stand in the way. For example, mammal migration along the lagoon could be disrupted. Also, the inflow of brackish waters from the Baltic sea could result in serious unbalancing of the lagoon's freshwater ecosystem.
From 1871 until 1919, the Lagoon was part of German Empire. Between 1919 and 1945 it was split between Germany and the Free City of Danzig. From January until March 1945 throughout the Evacuation of East Prussia refugees from East Prussia crossed the frozen Lagoon on their way to the west after the Red Army had reached the coast of the Lagoon near Elblag. Attacked by fighter aircraft thousands of them were killed or broke through the ice. Today the Vistula Lagoon is politically split between Poland and Russia's Kaliningrad Oblast.