or Odra River ancient Viadua
River, northern Europe.
It flows from its source in the Oder Mountains in the Czech Republic north through western Poland, where it forms the boundary between Poland and Germany. As the second largest river emptying into the Baltic Sea , it is economically important as a transport route. Navigable for 475 mi (765 km), it is connected by canal with the Vistula River and with the western European waterway system. It was partially internationalized under the Treaty of Versailles in 1919. Considered German until the 1945 Potsdam settlement, it was formally recognized as the Polish-German border in 1950 by East Germany and in 1970 by West Germany.