The flora and vegetation of river valleys are currently greatly transformed as a result of the increased influence of many factors related to human activity. Earlier research on the vegetation of the Rzeszow Reservoir was carried out in 1994-95. Since then, its depth has been greatly diminished and its surface area decreased two-fold. Moreover, types of land use have changed in the immediate vicinity of the reservoir. In 2011-14 we conducted similar field research aimed at analysing changes in vascular flora. For this purpose, we compared indices of anthropogenic changes in flora for both study periods. Our results show that the flora of this area is diverse in taxonomic, syntaxonomic, and ecological terms. It includes both typical aquatic plants and species characteristic for the habitats surrounding the water body. Currently, 327 vascular plant species of 68 families are recorded in the study area. The flora is dominated by hemicryptophytes, with high contributions of therophytes and hydrophytes. In the geographical-historical classification, the dominant groups were spontaneophytes (and among them apophytes), whereas the contribution of anthropophytes was small. This was confirmed by high values of synanthropization and apophytization indices (>50%), and the low value of the anthropophytization index (17.4%). The low index of flora naturalness (24.4%) attests to its transformation. The socio-ecological spectrum shows high contributions of species typical of alluvial forests: aquatic and tall fen communities (17%), fertile deciduous forests and shrub communities (14%), and fresh, moderately moist meadows (12%).