The Kolpa is a right-hand tributary of the Sava. It is a border river between the Republic of Croatia and the Republic of Slovenia with a total length of 292 kilometers, of which 113 follow the borderline. Its karstic spring lies in Croatia at an altitude of 313 meters. After the confluence with the Čabranka some kilometers after the spring, the Kolpa turns to the east and southeast. The Čabranka is a border river too, springing from a karstic exurgence on the Slovene side of the border in a picturesque Triassic dolomitic gorge just above the Cabar village which, in turn is on the Croatian side of the border. The Čabranka has a U-shaped river bed, is 15 kilometers long and runs on impermeable Permian stone ground, in which it has etched a relatively broad valley. It is a typical quick spring river. The Kolpa, on the other hand, features a relatively low gradient, running first in a canyon and then in a U-shaped valley. It cuts it is way between  high karstic plateaus and hills all the way to the Bela Krajina region, the only exception being the Kostel area, where it runs wide. In the Bela Krajina region the Kolpa features a U-shaped valley with up to 100 meters high banks, which area then gradually lowers to 20 meters. In the Griblje area the Kolpa enters a Ouarternary plain and is consequently spilled towards the town of Metlika. After that the Kolpa returns to a U-shaped valley, cutting its way through the southern foothills near Žužemberk heading east towards the Croatian village of Ozalj at an altitude of 126 meters.
Nearly all the area is karst, except for the river basin of the Čabranka and the Kostel area showing Permian argillaceous strata and sandstones. Where the valley and more recent strata of Triassic dolomite, Jurassic and Cretaceous limestone come together, many karstic springs and powerful exurgences can be found on both sides of the river, as it has numerous underground tributaries from the mountainous ranges of the Snežnik, Risnjak, Gorski Kotar and Kočevska Reka, from the Rinža river, the southern part of the Kočevski Rog range as well as the river basin of the Lahinja river and, naturally, from the Croatian karstic hills on the right-hand bank of the Kolpa.
The fact that the river has etched itself in the surface so deep is a consequence of strong Alpine - Dinaric uplift, as the Kolpa belongs to the river basin of the Panonic Sea ever since the Pliocenic levelling. Although the whole area streching from the Risnjak and the Gorski Kotar ranges on one side to the Velika gora, Stojna gora and the Goteniška gora mountains on the other, has a karstic character, the river etched itself deep in between these high karstic mountains, always remaining on the surface also due to the impermeable Permian strata in the Čabranka area. A regular hydrographic network ís therefore present only where the limestone strata were laid bare due to a tectonic uplift or where the river has etched itself deep enough to reach the Permian strata. The Bela krajina region is a combination of shallow karst and a large-scale levelling (the plateau of Ozalj - Slunj), a remnant of preceding levellings near the former Pannonia Sea. The largest Slovene surface tributary to the Kolpa is the Lahinja, which - although in the middle of karst - has also remained on the surface due to impermeable Pliocenic strata south of the Črnomelj town (at the Kanižarica).
The Kolpa is an extraordinary river for Slovenia: environmentally rather intact, picturesque and only poorly known. Along its whole length, we are beginning to appreciate its diversity, potential for recreational activities, abundance of fish and relaxation that comes from boating. The Kolpa is not strenuous to row and therefore also suitable for newcomers to rowing, but will be most cherished by people who love quiet calmness and camping near the river. However, it is best to go canoeing or kayaking after substantial rainfall, since the river has some 50 - odd milldams and weirs, witnesses of once numerous watermills, sawmills and forges that are today, sadly though, nothing but pleasant memories in most cases. Another important fact is that the Kolpa gets warm enough to make for a nice swim in the summer: swimming period lasts about a month in the upper stream and two months in the lower.