Despite the extensive research into hillslope and channel interactions in headwater catchments, surprisingly little attention has been paid to such processes in lowland rivers. In particular, previous studies have not addressed the influence of hillslope contributions and have concentrated solely on in-bank floods rather than more complex out-of-bank cases. Accordingly, we combine field monitoring and numerical modeling to study hillslope, floodplain, and channel interactions for a lowland river. Piezometric, precipitation, and river stage data were used to parameterize and test a new two-dimensional finite element model of saturated-unsaturated flow applied to two vertically aligned cross sections through a lowland floodplain. Data for two major out-of-banky flood events were simulated which appeared to show the presence of a significant unsaturated zone extending up to 5 m below the surface. The model simulated reasonably well the pressure head field that was recorded at a number of piezometers located internal to the computational domain on each transect, and we conclude that floodplain hydrology is predominately a two-dimensional (lateral) process. Three-dimensional (down reach) flow effects would seem to become more significant at the beginning and end of each event. The simulations also showed that the unsaturated zone remained close to saturation at all times and that it was not significant in terms of the floodplain hydrology. Examination of velocity vector patterns showed the formation of a strong groundwater ridge within the floodplain. This led to the development of strong velocities directed toward hillslope areas as the inundation front approached the hillslope/floodplain junction. This suggests that surface water may move into hillslope areas adjoining the floodplain during major floods. Thus the extent of the hyporheic zone may be larger than previously thought. ¿ 2000 American Geophysical Union