The subject of this final thesis are determinig the strength and duration of annual energy production of a hydropower plant, graphing the extent of usage of the river and construction, calculating the necessary accumulation for the daily peaking flow, as well as determining the needed installed capacity of the supplementary power plants in case the power plant operates at peak efficiency and in case it operates at base efficiency. The thesis goes through the basic terms used when discussing hydroelectric power plants, such as their basic components and the concepts used in the operation of hydropower plant. The classifications and features of hydroelectric power plants are also included, as well as descriptions of some of Croatia's and the world's hydroelectric power plants. At the end of the thesis is a practical problem concerning a run of the river hydropower plant.