The Kupa or Kolpa river, a right tributary of the Sava, forms a natural border between north-west Croatia and southeast Slovenia. It is 297km long, with its border part having a length of 118km and the rest located in Croatia.CourseThe 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.It then continues eastwards between the White Carniola region in the north and Central Croatia in the south. The Kupa receives influx from the river Lahinja from the left in Primostek, passes Vrbovsko, and eventually detaches from the Slovenian border having passed 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). 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.