Assessment of hydropower potential is the most important part of renewable sources balances. Quantification of the feasibility of hydropower plants realization is essential for estimating production of these kind of renewable energy sources. The quantitative assessment is most often based only of theoretical, technical and economic potential, which does not compare with real potential allowed to get from the river. The main scope of the paper is to show the course of action to estimate the effective potential of hydropower. Estimating the effective potential was done by research of actual legal procedures which allow the small hydropower plants realizations. The Upper Vistula water region in South Poland has been selected as an example area because of the highly recognized procedural conditions and many years’ experiences of the author. However the presented course of the analysis seems to be universal for investments in other regions in Poland and also in the other countries. The study case showing the main course, calculations and results of theoretical, technical and proposed effective potential of hydropower was based on Szreniawa river as an example. The resulting “effective potential of hydropower” provides the actual view on the hydropower generation capacity of the river and brings the actual values in prognosis and other computational appraisal. The comparison of results shown a very low value of actual potential with real possibilities of small hydropower plant realizations. The quantitative assessment based only of theoretical and technical potential of hydropower provides the overoptimistic results.