The paper presents phytosociological and hydrological results of field investigations carried out in old excavation in Mikołów - Gniotek town (the Silesian Upland) in June 2002. Owing to renunciation of building works, primary succession led to form riparian black alder forest Fraxino-Alnetum W. Mat. 1952 of anthropogenic origin. Vegetation of the area is mostly determined by water conditions indepentently of occurrence of sandy and silty clays in the soil.