The retention areas are used for temporary storage of water separated from the flow in the riverbed. This lowers the peak of the water wave and reduces the risk of flooding in downstream areas. Building retention along lowland rivers is a part of the strategy to combat the impact of global climate change and is projected along many European rivers. Generally, retention surfaces must be large enough to be purposeful. Furthermore, the construction of entrance and exit structures and embankments achieves the possibility of temporarily regulating flood defenses. This implies the need to designate large areas along lowland rivers as retention areas, in order to reduce the impact of large waters. This further implies the impacts on the ecological and economic functioning of such areas. The best example for studying the hydrological and ecological function of large artificial retentions in Europe is the Lonjsko Polje Nature Park. This paper examines the systems of defense against large waters, focusing specifically on the Srednje Posavlje and Lonjsko Polje.