Dams are major source of flow alteration and quantifying their impact is crucial from the point of view of the EU's environmental flow policy. This study demonstrates a method of assessing flow alteration by dams using a large-scale high-resolution hydrological model (SWAT) and three major Polish reservoirs as case studies. The results show that it has some advantages over more conventional methods, e.g. it allows for distinguishing between direct human effect and natural climatic effect.