CATASTROPHIC MEGAFLOODS IN THE ENGLISH CHANNEL

Megaflood events involving sudden discharges of exceptionally large volumes of water are rare but can significantly impact landscape evolution, continental-scale drainage patterns and climate change. It has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the English Channel, the narrow seaway between England and France. This hypothesis has remained untested through lack of direct evidence, and alternative non-catastrophist ideas have been entertained for valley formation. We present a regional bathymetric map of the English Channel derived from high-resolution sonar data, which shows the morphology of the valley in unprecedented detail. We observe a large bedrock-floored valley that contains a distinct assemblage of landforms, including streamlined islands, longitudinal erosional grooves, and cataracts, which are indicative of large-scale subaerial erosion by high-magnitude water discharges. These landforms are comparable to features associated with catastrophic flooding in the Channeled Scabland in the western USA. Our observations support the megaflood model, in which breaching of a rock dam at the Dover Strait during a glacial lowstand of sea-level instigated catastrophic drainage of a large pro-glacial lake in the Southern North Sea basin. We suggest that megaflooding provides an explanation for permanent isolation of Britain from mainland Europe during interglacial high sea-level stands and consequently patterns of early human colonisation of Britain together with the large-scale reorganisation of palaeodrainage in NW Europe.