Water level positions during the Subatlantic in the coastal plain of Belgium and Northern France are assessed by means of indicators such as tidal levees, upper tidal flat deposits and mature salt marshes. The indicators are evaluated for the reconstruction of local mean high water and local mean spring high water levels in the intracoastal zone. No significant altitude difference is observed between the indicators genetically related to the Dunkerque II inundation phase, i.e. before the land reclamations of the 10th century AD, and the medieval Climate Optimum (AD 1000-1350). There is no evidence for a high relative local water level or a water level rise during the medieval Climate Optimum. It is shown that a hydrological phenomenon, probably an estuary effect, caused water levels to rise significantly in restricted areas of the former tidal flat during the Dunkerque II phase. A comparison between the local water level record and the tide gauge records at Oostende indicates a more marked fluctuation over the last 160 years than during the preceding 700 years