River entrenchment and terrace formation in the UK Holocene M.G. Macklin a, * , J. Lewin a , A.F. Jones b a Centre for Catchment and Coastal Research and River Basin Dynamics and Hydrology Research Group, Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion SY23 3DB, UK b School of Geography, Planning and Environmental Policy, University College Dublin, E001 Newman Building, Belfield, Dublin 4, Ireland article info Article history: Received 17 November 2012 Received in revised form 15 May 2013 Accepted 17 May 2013 Available online Keywords: River terraces River incision Holocene Flood events Environmental controls abstract A meta-analysis of a large database of 14 C 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 14 C 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 (0.5e3.5 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 4200e3700, 3100e2900, 2100e1900, 1800e 1500 cal. BP and most notably the last 1000 years (with prominent peaks at 900e800 and 700e 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. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Meta-analysis of the UK Holocene fluvial archive that began more than 20 years ago has demonstrated the episodic nature of river alluviation (Macklin and Lewin, 1993, 2003). It has also shown that fluvial units which mark changes in the style of sedimentation cluster into time periods and coincide with phases of environ- mental change (Macklin et al., 2005, 2010, 2012a; Johnstone et al., 2006) controlled by climatic fluctuations and human activities. This approach has been successfully adopted in mainland Europe, (Macklin et al., 2006; Starkel et al., 2006; Thorndycraft and Benito, 2006; Hoffman et al., 2008), North Africa (Zielhofer and Faust, 2007), the Indian subcontinent (Kale, 2007), the American South- west (Harden et al., 2010) and most recently in New Zealand (Macklin et al., 2012b; Richardson et al., 2013), and is transforming our understanding of Holocene river dynamics and its controls. Underlying this new methodology is the long-held (e.g. Macklin et al., 1992a,b; Macklin, 1999) realisation that single-reach studies usually contain only a partial picture of Holocene alluviation, both because the quality of geochronologies depends on, and partly re- flects, the sedimentary environments available within a single site (Lewin et al., 2005) and the degree to which these have been pre- served from later erosion and removal (Lewin and Macklin, 2003). However, what has not been systematically evaluated either at a multi-reach and particularly at a multi-catchment scale is the na- ture and timing of Holocene river entrenchment, and the factors that control long-term and large-scale incision in alluvial rivers. In this paper meta-analysis of a large database (826) of 14 C-dated Holocene fluvial units from the UK is used for the first time to address this meso-form development problem, and to try to better understand river terrace development more widely. It has two primary objectives: 1. to establish the number, magnitude, duration and timing of Holocene fluvial incision episodes in UK rivers; and 2. to evaluate the short and long-term controls of Holocene river incision, including changing flooding regimes (resulting from climatic fluctuations and human modifications of catchments), * Corresponding author. Tel.: þ44 (0) 1970 622656; fax: þ44 (0) 1970 622659. E-mail address: mvm@aber.ac.uk (M.G. Macklin). Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.05.026 Quaternary Science Reviews 76 (2013) 194e206