Catastrophic Flooding and the Origin of the English Channel Valley System

Late Quaternary episodes of sea-level lowering exposed large expanses of the English Channel shelf during glacial periods leading to the development of an English Channel mega-river system that integrated major north-western European rivers such as the Rhine and Seine with the Thames and flowed to the Celtic Sea continental margin. On the eastern English Channel shelf, extensive bedrock-incised valleys form an anastamosing network, however the controls on their genesis and evolution remain enigmatic due to a lack of detailed morphological data. Suggested mechanisms for valley incision include fluvial erosion in response to sea-level lowering, tidal or glacial scouring, and controversially catastrophic flooding. Here, we present new data on the detailed morphology of the northern branch of the valley system, the Northern Palaeovalley that shows clear evidence for catastrophic flooding as the cause of valley incision. By analysing a bathymetric grid derived from singlebeam sonar data compiled from UK Hydrographic Office data we have identified an assemblage of geomorphic features associated with the valley that are analogous to features indicative of catastrophic flood scouring. The valley is tens of kilometres wide and up to 50 m deep, and shows distinctive box-like cross-sections. Characteristic landforms observed include longitudinal erosional grooves, streamlined islands, inner gorges and erosionally terraced and smoothed, streamlined valley margins. These geomorphic features bear striking resemblance to landforms observed in catastrophic flood terrains in the Channeled Scabland and within outflow channels on Mars, and are difficult to explain by normal fluvial, tidal or glacial erosion mechanisms. Our results provide the first evidence indicating that the English Channel palaeovalley system was carved by catastrophic flood flows following breaching of the Dover Straits and abrupt release of freshwater from an ice-dammed lake in the present Southern North Sea region. We speculate that the English Channel flood may have been one of the most powerful flood events on Earth.