Hydrologic gradients that can be established for each alluvial river valley promote the diversity of wetland communities and community types. In case of swamps, long-lasting inundation is an essential hydrological phenomenon, that shapes the habitat characteristics. Considering natural hydrological regime and relative small hydromorphological changes in the Lower Biebrza River valley (NE Poland), three typical swamp ecosystems along its floodplain were analysed. Those were: reeds, sedges and sedge-moss ecosystems.

The one-dimensional (1D) unsteady hydrodynamic model was combined with the Digital Elevation Model (DEM) to determine flood extent in the river valley. Then, the spatial relation between daily inundation maps and vegetation maps was analyzed. As a result, the frequency of flooding and the average duration of flooding for each community was identified for the 50 years historical water stage data. The critical analysis of method accuracy led to estimation of average frequency of flooding for different wetland ecosystems.