The Kupa (Croatian) or Kolpa (Slovene, from Latin: Colapis in Roman times) river, a right tributary of the Sava, forms a natural border between north-west Croatia and southeast Slovenia. It is 297 kilometres (185 miles) 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 Čabranka River from the left, before reaching the Slovenian border.
In its lower course, Kupa meanders through partly forested areas near Karlovac.
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). The Kupa continues flowing to the east, where it merges with Glina from the right as well as Odra from the left, and proceeds to the town of Sisak where it flows into the Sava River.
Fairly unpolluted downstream to Karlovac, the upper Kupa is a popular place for bathing in summer. The section from Stari Trg down to Fučkovci since 2006 is part of the Slovenian Krajinski park Kolpa nature reserve.