It is included in the list of so called Ramsar sited of international importance, especially as a habitat of wetland birds. According to the criteria from the European Union Birds Directive, this area is an Important Bird Area – IBA. It is mostly made up of three fields (polje): Lonjsko, Mokro, and Poganovo polje.

The rivers Sava, Una, Kupa, Lonja, and Strug meet in the nature park. Due to their very complex dynamics, flooding in this area is very unpredictable, so it must be considered as a possibility throughout the year. The water level of Sava downriver from Sisak can rise up to 10 m! If the water level in all the mentioned rivers rises suddenly, large amounts of water meet and cause flooding. The central Posavina area can accept an enormous amount of two billion cubic metres of water. So the water rules over Lonjsko polje and even the local population have accepted its rule.

The rivers Sava, Una, Kupa, Lonja, and Strug meet in the nature park. Due to their very complex dynamics, flooding in this area is very unpredictable, so it must be considered as a possibility throughout the year. The water level of Sava downriver from Sisak can rise up to 10 m! If the water level in all the mentioned rivers rises suddenly, large amounts of water meet and cause flooding. The central Posavina area can accept an enormous amount of two billion cubic metres of water. So the water rules over Lonjsko polje and even the local population have accepted its rule.