Floodplain Change in the Glenmalure Valley, South-east Leinster

Abstract

The Glenmalure Valley is one of several U-shaped valleys which drain the eastern flanks of the Wicklow Mountains, 45km south west of Dublin. Five channel and floodplain reaches are recognised along a 25km stretch of the Glenmalure River on the basis of spatial changes in stream power and sediment load characteristics. The downvalley sequence of floodplain types within the river is atypical of that commonly associated with decreasing slope and sediment calibre downstream. Radiocarbon dating together with the use of anomalous concentrations of heavy metals, which are used as stratigraphic indicators of alluvial deposition post 1800, also reveal temporal changes in the rate of Holocene floodplain development within the valley. Downstream changes in floodplain processes within the valley are explained, therefore, in terms of spatial and temporal changes in the river's ability to adjust its slope, form and bed texture within the constraints of geological and Pleistocene geomorphic inheritance. The presence of a bedrock basin in the valley floor profile, infilled by Pleistocene lacustrine deposits, is believed to have caused a local change in base level leading to a significant reduction in valley slope and local stream power in the middle reaches of the valley. The subsurface bedrock form and the Pleistocene palaeolake also had a significant impact on coarse sediment transport, storage and transfer through the river system and hence on the ability of the Holocene river to regrade its slope and laterally rework Pleistocene valley fills for floodplain construction.