The source of Kupa is a small karst lake of turquoise green colour located near the village of Razloge in Gorski Kotar. Approximately 100 m downstream on the right-hand side it receives water from the occasional torrential stream of Krašićevica, and further down on the left side from the occasional stream of Sušica, and turns north-east, and then north further downstream. Further downstream, Kupa is a fast river, but after a couple kilometres it loses its speed and slows into a calm river, interspersed by numerous weirs that were once used for powering watermills: flour mills and sawmills. Kupa turns into a border river at the location where the river Čabranka flows into it from the left-hand side. Kupa runs wild in its upper part, which is why boat rides here offer a true adrenaline experience, and its clean and crystal-clear waters are nice for swimming during the summer months.
In its upper reaches, Kupa makes its way through a wooded canyon. The canyon widens in places, making space for fertile agricultural land. The source of the river Kupa, as one of the many still unsolved mysteries of the karst landscape, is one of the strongest, widest and deepest springs in Croatia. At 313 m above sea level, the body of water formed a small lake of turquoise green and blue colour, 200 metres in length and averaging at around 30 metres in width, from which water flows at the full width of the river. The source lies below Gavranova stijena, a vertical and almost straight-cut cliff around 250 metres tall. Access to this site is possible via a scenic serpentine trail from the village of Razloge or the village of Kupari, where the path is somewhat longer, and climbing upwards, parallel with the flow of the river, or from the valley of Kupa, from the village Hrvatsko.
As the second largest river of Gorski Kotar, Dobra River forms east of Skrad, from a number of smaller headstreams at Bukov Vrh, joined by Kamačnik near Vrbovsko. Dobra is a subterranean river, and its valley consists of broadenings and gorges. Near Ogulin, its flow disappears in the remarkable Đulin Ponor, and then emerges again some dozen kilometres downstream in the form of a powerful spring (Gojačka Dobra). It features two watermills still in operation and two hydroelectric power plants, and abounds in various species of fish.
Čabranka, the left tributary of Kupa, measures only 15 km in length and emerges from the cracks of the steep rocks of Veliki Obrh (546 m), located about a fifteen-minute walk from the centre of Čabar. Throughout its entire course it acts as a border river between Slovenia and Croatia.
It owes its stunning appearance, right from its source, to its numerous waterfalls and rapids. It was recognised as a protected area in 1961.
The source of the underground river Gerovčica emerges beneath a 300 m tall cliff. It never dries up, and in the summer, it doesn’t flow in the Gerovo valley. The river's course is marked by scenic rapids and beautiful natural landscapes.