The rivers of the lowland area belong to the Black Sea basin. They have a mild slope, a calmer flow, are longer and richer in water. The longest and most important rivers are Sava, Drava, and the Danube. The rivers of the Adriatic basin have a shorter flow, but with steeper slopes. The most important rivers in the Adriatic basin are Neretva, Cetina, and Krka. One of the most attractive locations for filming in Croatia is Zrmanja river. The largest natural lake in Croatia is Lake Vrana (nature park) near Biograd (area 30.7 km2), not far from the sea, while the most famous are Plitvice Lakes (National Park), part of the UNESCO World Heritage List.