Oder

The Oder is a river in Central Europe. It is Poland's second-longest river in total length and third-longest within its borders after the Vistula and Warta. The Oder rises in the Czech Republic and flows 742 kilometres through western Poland, later forming 187 kilometres of the border between Poland and Germany as part of the Oder–Neisse line. The river ultimately flows into the Szczecin Lagoon north of Szczecin and then into three branches that empty into the Bay of Pomerania of the Baltic Sea.

Source: Wikipedia