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.