Production of the electric energy from the rivers by using the mini, as well as micro, hydroelectric power plants is a very prospective solution, especially in the rural and isolated areas. Numerous waterways in Croatia and their hydrological and hydrogeological diversity present an opportunity, but also a challenge, for the construction of hydroelectric power plants. Due to the complexity of the water courses hydrology, as well as hydrogeological characteristics, it is very hard to determine an appropriate flow pattern (amount), which will be used as an input value for the sizing of hydroelectric power plants. Such analysis will be provided for real case studies in Croatia with special regard to present geological media - media with intergranular porosity (river Bednja), karstmedia (river Gornja Dobra), and flysch media (river Mirna). Taking into account different geological media increases the possibility of using the presented methodology on other locations in Croatia, as well in the world.