Nature Park as a wetland area is among the most threatened habitats in the world. It is the largest protected wetland area, not only in Croatia but also in the Danube basin. Was included in the so-called. Ramsar List of Wetlands of International Importance, especially as a habitat for waterbirds. According to the criteria of the Birds Directive European Union, this area belongs to the important areas for birds (Important Bird Areas - IBA). It consists of mainly three fields: Lonja, wet and Poganovo field.

Water - the ruler of the Lonja field

Rivers Sava, Una, Kupa, Lonja, and Strug meet precisely at the nature park. Because of the very complex dynamics in this area of the flooding is unpredictable and to them must be reckoned with at any time of year. Save water level downstream of Sisak and can grow to 10 m! If all these rivers appear suddenly high water, large amounts of water are colliding and causing flooding. District Posavina can accept a huge amount of two billion cubic meters of water.So Lonja field of water becomes the ruler who was denied even a man.