The Kupa or Kolpa river, a right tributary of the Sava, forms a natural border between north-west Croatia and southeast Slovenia. It is 297 kilometres long, with its border part having a length of 118 km (73 mi) and the rest located in Croatia.

The Sava is a river in Central and Southeast Europe, a right-bank and the longest tributary of the Danube. It flows through Slovenia, Croatia and along its border with Bosnia and Herzegovina, and finally through Serbia, feeding into the Danube in its capital, Belgrade. The Sava forms the main northern limit of the Balkan Peninsula, and the southern edge of the Pannonian Plain.