The Kupa (Serbo-Croatian) or Kolpa (Slovene) river, known in Roman times as Colapis (Latin), forms a natural border between north-west Croatia and southeast Slovenia. It is 297 kilometres (185 mi) long, with its border part having a length of 118 km (73 mi) and the rest located in Croatia.
The Kupa originates in Croatia in the mountainous region of Gorski kotar, northeast of Rijeka, in the area of Risnjak National Park. It flows a few kilometers eastwards, receives the small river Čabranka from the left, before reaching the Slovenian border.
It then continues eastwards, receives influx from the river Lahinja from the left in Primostek, passes Vrbovsko, and eventually detaches from the Slovenian border in Metlika. It then reaches the city of Karlovac, where it receives influx from two other rivers from the right, Dobra and Korana (which in turn is joined by Mrežnica).
It continues flowing to the east, where it merges with Glina from the right as well as Odra from the left, and proceeds to the city of Sisak where it flows into the Sava.
The hydrological parameters of Kupa are regularly monitored at Radenci, Kamanje, Karlovac and Jamnička Kiselica.