Geomorphology and palaeoecology of late Holocene floodplain environments in the river Irthing, Cumbria, UK

Abstract

Palaeochannel development along the middle reaches of the River Irthing valley floor, Cumbria, UK, has been reconstructed via a range of palaeoenvironmental techniques. This has enabled the analysis of interactions between the geomorphological, hydrological and ecological components of the floodplain system during the mid to late Holocene. Geornorphological mapping, surveying and lithostratigraphic analysis of palaeochannel fills along a 2.5 kilometre reach of the valley floor has determined the character of Holocene channel and floodplain evolution and the physical context for palaeochannel habitat development. Chronological controls have been provided by historical map analysis and radiocarbon dating. The River Irthing valley floor experienced net fluvial incision from the Pleistocene-Holocene boundary up until the mid Holocene, while late Holocene floodplain evolution has been characterised by a series of channel avulsion and limited floodplain reworking. Periods of channel incision and planform change have been dated to 2440-1920 cal. BC, 670-970 cal. AD, 1410-1620 cal. AD and the late eighteenth to early nineteenth century AD. Five palaeochannel reaches with well-preserved organic rich channel fill sediments were selected for detailed lithostratigraphic analyses (by multiple core transects) and plant macrofossil analyses. Channel fills provide evidence of rapid biotic response to channel abandonment and subsequent changes to the physical characteristics and trophic status of the habitat. Hydroseral sequences from aquatic to wetland to floodplain woodland communities and the affects of human activity on palaeochannel development are also evident. The results indicate high magnitude flood inundation significantly affects vegetation succession and highlight the importance of physical processes and the landscape context in determining the characteristics of palaeochannel development. The research emphasises the application of plant macrofossil analysis to organic rich alluvial sediments.