The Vistula is the only big European river that has retained its natural character, and that is why it has been chosen for a model in projects aiming at re-naturalising the deeply transformed rivers of Western Europe. The paper deals with research on the diversity of the flora in the middle Vistula valley between the mouth of the San and the Wieprz rivers. The paper presents the relations between the flora and selected environmental factors (denivelations, surface features) and the degree of changes in vegetation resulting from the intensity of land use (hemeroby, fragmentation of habitats).The number of species and their diversity is the highest on valley edges, where there is a mosaic of communities of various hemeroby: from xerothermic grasslands and shrubs to ruderal communities. Valley edges are also characterised by strong fragmentation. The lowest number of species and little diversity has been observed in the areas immediately next to the river channel. Foreign species constitute a considerable percentage of plants on lower river terraces. The present-day diversity results from the various forms of land use and diverse surface features rather than the activity of the river. The situation has been caused by building the embankments that divide the valley.