Kupa springs from a turquoise-green karst lake in Risnjak National Park, near Razloge in Gorski kotar. About 100m downstream, on the right side, it receives water from the occasional torrential stream of Krašićevica, and further on from the left side, the occasional stream of Sušice, and turns to the northeast, and then to the north. Below the source of the Kupa is a fast river, while after a few kilometers it calms down and becomes a calm river intersected by many artificial waterfalls that in history were used to power watermills: mills and sawmills. The border river Kupa stops at the point where the Čabranka flows into it from the left. In the upper reaches of the Kupa, it is partly untamed, so boating in this part offers real adrenaline pleasure, and its clean and crystal clear water is pleasant for swimming in the summer months.