Hydrological disasters in the NW-European Lowlands during the first millennium AD: a dendrochronological reconstruction

Abstract This study presents an annually resolved dendrochronological reconstruction of hydrological impacts on the Roman and early-medieval landscape in the Low Countries of northwestern Europe. Around 600 hydrologically sensitive ring-width patterns, mostly oak (Quercus robur/petraea) as well as some ash (Fraxinus excelsior) and elm (Ulmus sp.), were selected from an initial dataset of >5000 and compiled into two chronologies that span the first millennium AD. Their content and (dis)similarities to established tree-ring chronologies from this and surrounding regions were used to assess their provenance, which in both cases is in the area where the majority of the wood was recovered. Instances of high groundwater levels and/or inundation were catalogued by identifying multi-year intervals of strongly reduced annual growth that occurred simultaneously throughout the research area. The resulting record contains 164 events dated between AD 1 and 1000, of which 21 have a recurrence frequency ≥50 years. One-third of the ≥50-yr events date between AD 185 and 282, making this the most flood-intense interval of the first millennium. The severest reconstructed impact of the first millennium dates to AD 602. A comparison to historically documented river floods/sea breaches and drought/heat spells shows that the predominant cause of the inferred impacts in the research area was river overflow. Synchronous inundation responses of oaks preserved in former bogs in Lower Saxony (NW Germany) indicate that half of the reconstructed events occurred on a supra-regional level, pointing to regional precipitation as a main forcing. River floods documented in written sources do not seem to have affected tree growth in Lower Saxony in a significant manner, indicating that the majority of documented floods most likely were caused by hydrological circumstances upstream of the catchments of the Rhine and/or Meuse. Reconstructed flood impacts during the Early Middle Ages coincide remarkably well with construction and repair of Rhine revetments at the early-medieval site of Leiderdorp-Plantage in the western Netherlands.