Hydropower production is the most important renewable energy source in the Italian electricity system, with more than 2.000 plants, more than 17 GW power installed and a mean annual production of about 39 TWh. Due to the increasing attention to environmental protection on the one hand, and unexploited watercourses on the other hand, the role of small hydropower has gained particular attention. Recent studies concerning the potential of small scale hydropower generation in Italy show that there are conditions for increasing the current level of small hydro production [2][5]. One aspect is related to the “remaining” hydropower potential, which takes into account the current water resource exploitation (irrigation, drinkable water, existing hydro power, etc) and the minimum environmental flow. In the present paper the next step of the method is developed to calculate the “effective” hydropower potential: there are technical and environmental constraints that cut down the quantity of energy that could be possible to recover. Constraints like protected areas, elevation, populated zones and others are considered in this work. The GIS model covers the whole Italian territory with a distributed concept using 90×90 m grid. Each cell has the necessary physiographic information, the rainfall data, the correspondent runoff coefficients, the remaining discharge and the effective potential energy. The model is applied to a case study in the Sinni River (Basilicata – Italy), taking into account the present water exploitation scheme and the technical and environmental constrains as national and regional parks. The effective hydropower production map demonstrates to be a quite helpful tool to support administrations, decision makers and stakeholders in general, to make Energy Master Plans, or assess and implement small scale hydropower plants..