Time Out says
Swim four rivers in one day
Starting with the Korana, where Foginovo Beach has been a designated bathing spot for generations, a visitor keen on active sports may spend the day in the waters of the four rivers that meet at Karlovac. The Mrežnica is often tackled by teams of kayakers, the Dobra powers old mills further south and the Kupa has its source in the Risnjak National Park before flowing past the centre of Karlovac. Ask a trusty local, and he will tell you at which point you can swim in all four of these rivers by only diving in twice.