Kolpa River is one of the cleanest and warmest freshwater rivers in Europe and it offers an excellent open water swimming destination in the summer months. Kolpa springs from a small turquoise karst lake in the Croatian Risnjak National Park at the mountainous region of Gorski Kotar. The river source is not far away from the Adriatic sea in Istria. As the river flows east towards the Danube river and Black sea it forms a natural border between Croatia and Slovenia. The total river length is 292km with the Slovenian part stretching 113km.