The hydropower sector has gained more importance in the last decade since renewable resources have recorded a worldwide investment boom. Based on its advantages referring to the timely response at peak energy demand with excellent flexibility, hydropower plants must operate as efficiently as possible to cover the energy variations produced by the renewable sources. In the paper, the data mining techniques integrated into a special Knowledge Discovery module of the SCADA system have been proposed to assist the Decision-Maker (DM) in the optimal operation of the hydropower plants. The main advantages refer to the high efficiency of the data processing process and shortening the time interval in which the DM makes the decisions.