Flood alluviation and entrenchment: Holocene valley­floor development and transformation in the British uplands

The morphology, sedimentary properties, and sequence of recent coarse­grained flood deposits and earlier Holocene alluvial fills were investigated in Thinhope Burn, a small (12­km2) catchment in the Northern Pennine uplands, northern England. Twenty­one large flood events are recorded by distinctive cobble­boulder bars, sheets and splays, and boulder berms and lobes. Lichenometric analysis showed that all but one of these floods dated from the mid­eighteenth century. The timing of large floods between 1766 and 1960 corresponds with major hydroclimatic trends evident in northern Britain and northwest Europe over this period. Discharge estimates suggest that flood magnitudes have decreased since the mid­eighteenth century. Channel and flood­plain metamorphosis in late Roman times and in the eighteenth century, following major valley­floor entrenchment (locally as much as 8 m), would appear to have been caused by increased runoff and flood magnitude. This was linked to a shift to a wetter climate with flow augmented by Iron Age and Roman woodland clearance, and drainage of the catchment in more recent times. Results from this study suggest that current models of longer­term Holocene and Pleistocene valley­floor development in the British uplands may need to be re­evaluated.