The potential for paleolimnology to determine historic sediment delivery to rivers

Establishment of water quality criteria to guide catchment sediment management is required by the European Union (EU) Water Framework Directive. The topic, however, is hotly contested among scientists and policy makers. Existing legislation with regard to fine sediment was set by the EU Freshwater Fish Directive. Its guideline, i.e. mean annual suspended sediment concentration, is 25 mg l(-1). Such a static target fails to capture the episodic nature of sediment transport. Furthermore, application of such global standards is inappropriate for a pollutant that is strongly controlled by spatial variation in key catchment drivers. Paleolimnology offers an approach for assessing background sediment pressures on watercourses, enabling determination of values for times pre-dating agricultural intensification. We propose that Modern Background Sediment Delivery to Rivers (MBSDR) across England and Wales can be determined using paleolimnology to quantify maximum feasible sediment reduction. No management programme should aim to reduce sediment loss to values below those resulting from background, natural physiographic and/or hydrological controls. Lacking generic tools to quantify process linkages between sediment pressures and biological impact, we propose that MBSDR could be taken to represent ecological demand for sediment inputs into watercourses required to support healthy aquatic habitats. In situations where generic tools exist for coupling sediment pressures and ecological impacts, assessment of MBSDR could be used to correct the gap between current or future projected sediment loss and biological condition. Existing paleolimnological data on sediment yields across England and Wales are presented to illustrate the approach and provide preliminary national estimates of MBSDR. We briefly consider the basis for reconstructing sediment yields using a paleolimnological approach and analyse temporal trends in published sediment yield, inferred for a range of landscape types. We also attempt to correlate sediment accumulation rates (SARs) with sediment yields to extend the MBSDR data base. Preliminary maps were generated to identify regions where further sediment yield data are needed to produce a more robust estimate of the spatial distribution of MBSDR across England and Wales.