This study presents the problematics of controlling floods caused by the river Sava in ˝srednje Posavlje˝ area through retentions. The goal is to relieve the flood wave in its retention area and then to discharge it in a controlled way. Retentions of the“ srednje Posavlje“ area on the Sava river are: Odransko polje and Zelenik on the right side and Žutica, Lonjsko polje, Opeka, Trstik and Mokro polje of the left side. The work is divided into several sections. The introduction defines basic problematics of flood control and general problematics of the paper itself. In the second section, characteristics of flood control are explained, how the system itself changed and developed through history and which are the main parts of flood control system through retentions. The third section deals with retentions in general but also describes each retention's general purpose on the river Sava when dealing with flood control on the srednje Posavlje area. The fourth section deals with analyses showing whether retention Mokro polje could or could not accept flood waves flooding its area in given periods of time. At the end of the study there's a conclusion showing an overview of the work as a whole.