The Kupa or Kolpa (Slovene) river forms a natural border between north-west Croatia

Croatia

Croatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...

 and southeast Slovenia

Slovenia

Slovenia , officially the Republic of Slovenia , is a country in Central and Southeastern Europe touching the Alps and bordering the Mediterranean. Slovenia borders Italy to the west, Croatia to the south and east, Hungary to the northeast, and Austria to the north, and also has a small portion of...

.

The Kupa originates in Croatia in the mountainous region of Gorski kotar

Gorski kotar

Gorski kotar is the mountainous region in Croatia between Karlovac and Rijeka. Together with Lika and the Ogulin-Plaški valley it forms Mountainous Croatia. Because 63% of its surface is forested it is popularly called the green lungs of Croatia or Croatian Switzerland...

, northeast of Rijeka

Rijeka

Rijeka is the principal seaport and the third largest city in Croatia . It is located on Kvarner Bay, an inlet of the Adriatic Sea and has a population of 128,735 inhabitants...

, in the area of Risnjak National Park

Risnjak National Park

Risnjak National Park is a national park in Croatia. It is located in Gorski kotar, the most mountainous and heavily forested region of the country, about 15 km inland from the Adriatic Sea. The park covers an area of 63.5 square kilometers including the central part of Risnjak and Snježnik...

. 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

Lahinja

Lahinja is a river in White Carniola, the extreme southeastern part of Slovenia. It is long and originates from several karstic sources at the end of a steephead valley between the villages of Knežina, Belčji Vrh, and Mali Nerajec....

 from the left in Primostek

Primostek

Primostek is a village in the Metlika municipality in the Bela Krajina area of southeastern Slovenia. It lies on the right bank of the river Lahinja, close to its confluence with the Kolpa, next to the border with Croatia. The entire area is part of the traditional region of Lower Carniola and is...

, passes Vrbovsko

Vrbovsko

Vrbovsko is a town in western Croatia, situated at the far east of the mountainous region of Gorski kotar in the Primorje-Gorski Kotar county; on its 280 square kilometers area, Vrbovsko features 60 settlements and a total of 5,019 inhabitants...

, and eventually detaches from the Slovenian border in Metlika

Metlika

Metlika is a town and municipality in the southeastern Slovenia. It lies on the left bank of the river Kolpa on the border with Croatia. The municipality is at the heart of the area of Bela Krajina, the southeastern part of the traditional region of Lower Carniola. It is now included in the...

.