HOLOCENE WATER LEVEL DEVELOPMENT IN THE NETHERLANDS RIVER AREA - IMPLICATIONS FOR SEA-LEVEL RECONSTRUCTION

Plotting of radiocarbon datings of samples from the base of peat layers on the flanks of fossil river dunes results in time-depth graphs that can be interpreted as curves of the local rise of the groundwater table. Combination of data from individual sites makes it possible to reconstruct groundwater gradient lines at selected moments. All gradient lines appear to consist of a steep upper part (river dominated) and a level lower part (sea level dominated). Between 6750 and 2850 BP the knickpoint between the two wandered some 10 km upstream. During all of the Holocene the area east of Leerdam remained outside the direct influence of the sea. Irregularities in individual curves in the downstream area-if not caused by dating errors - may represent varying rates of MSL rise, varying tidal range and a varying floodbasin effect. For the pre-5000 BP period, assumed sea level dominance in the downstream area is not consistent with the general evidence on the position of the sea level in The Netherlands. This situation is possibly due to less tectonic subsidence in the southwestern coastal sector.