Water balance of wetlands within lowland floodplains is strongly influenced by the temporally variable spatial extent of the interactions between groundwater and surface water. A robust algorithm will be introduced which makes it possible to delineate the interaction zone between the lowland river and the floodplain. This interaction zone is specified as the "Direct Catchment" which is defined by the part of the connected floodplain in which wetland water balance is mainly affected by the surface water dynamics of the adjacent river. The delineation algorithm is based on transfer functions which were assessed by local simulation results of the integrated water balance and nutrient dynamics model IWAN. The transfer functions are further determined by mean annual groundwater depths and by simulated groundwater dynamics. They are controlled by simulation results of the maximal transversal extent of surface water influence on groundwater stages. The regionalisation of the developed delineation algorithm leads to the specification of the maximal extent of groundwater - surface water - interaction processes along the river. By application of this approach to the Havel River basin, located within lowlands of Northeaster Germany, it was possible to specify a 998.1 km² part of the floodplain which is directly connected with the surface waters and thus called the "Direct Catchment" of the Havel river. The IWAN model was applied to simulate the water balance of the floodplain. The simulation results prove the tight interaction between river and floodplain. It is shown that the spatially and temporally variable influences of the connected floodplain on the river discharge were only important during low discharge in summer.