River response to rapid Holocene environmental change: evidence and explanation in British catchments

Since the late 1980s our understanding of the relationship between climate, land-use and Holocene river development in Britain has changed radically. Conceptually simplistic and data-poor models of people–river environment interaction, many of which considered human activity alone to be the control of Holocene flooding and alluviation, have been replaced by more data-rich models that explain river behaviour in terms of a continuum of climate and anthropogenic drivers, individually operating over a range of temporal and spatial scales. One of the key factors in this paradigm shift has been the significant increase in the number of 14C-dated fluvial units that has itself recently facilitated the application of meta-analysis techniques to large fluvial data sets. These analyses have produced the first probability-based national and regional reconstructions of Holocene flooding in the British Isles that can be correlated with independent, high-resolution hydro-climate records. This paper critically reviews the major findings of this new research focusing on process-based causality relationships in fluvial systems and quantifying how environmental signals are propagated in river basins and preserved in landform and sedimentary sequences. By using cumulative probability density function (CPDF) plots of fluvial 14C dates that have been classified on the basis of sedimentary environment and evidence for changing depositional regime, 17 multi-centennial length periods of flooding and river instability during the Holocene are identified in the UK. The scale and timing of these episodes vary regionally and correlations can be made with a range of climate proxy records, including the North Atlantic drift-ice index, glacier variations in Europe, and mire surface wetness and water-table reconstructions. A new analysis of overbank floodplain sedimentation rates points to an acceleration after ca 1000 cal. BP, which can be related to the agricultural revolution of the Middle Ages. This is the first time that an unequivocal anthropogenic signal, affecting most British rivers, can be recognised in the Holocene fluvial record of the UK and it appears to have been a broadly synchronous ‘event’ in many catchments.