River entrenchment and terrace formation in the UK Holocene

A meta-analysis of a large database of 14C dated fluvial units is used to assess the chronology and controls of episodic Holocene river entrenchment and terrace formation in the UK. Most Holocene terraces are of a 'fill-cut' type developed in Pleistocene sediments, in places now reaching down to pre-Holocene bedrock. Holocene terraces are widespread in higher-relief areas of the UK and peripheral higher-energy rivers, and include up to 7 levels in some valley floor reaches. Using 14C constrained data sub-sets for incision episodes, the onset, vertical ranges, formation times, and rates of entrenchment are examined, together with geographical distributions. The height range of terrace separation is relatively small (0.5-3.5 m) with a long-term averaged incision rate over the late and mid-Holocene of 0.43, 0.5, 0.67, 0.7 and 0.81 m/ka in the Tweed, Rheidol, Severn, Ouse and Ribble catchments, and a regional similarity in the scale of incision events. The periods 4200-3700, 3100-2900, 2100-1900, 1800-1500 cal. BP and most notably the last 1000 years (with prominent peaks at 900-800 and 700-600 cal. BP) were times of accelerated incision. It appears likely that extreme flood events triggered the formation of incision 'slots', rather than entrenchment being a direct response to glacio-tectonic uplift, or the result of incremental valley-floor lowering by combined incision and lateral reworking. River entrenchment has also been rapid in recent centuries, reflecting the coupling of extreme-events with anthropogenic effects on catchment hydrology. Incision results in changes of river channel flood power and overbank flood extent, and improved data on the long-term and large-scale vertical tendency of UK rivers are needed for flood risk management purposes.