Wildfire Effects on Suspended Sediment Delivery Quantified Using Fallout Radionuclide Tracers in a Mediterranean Catchment.

Over short and long timescales, wildfires can be an important cause of hydrological and geomorphological change. Mediterranean rivers are part of a fire-prone and high-energy environment in which the timing of major storms in relation to fire influences the impact on fluvial systems; accordingly, the identification of major sources, stores and fluxes of sediments is essential for providing more effective post-fire management strategies. In this study, caesium-137 and excess lead-210 were used as tracers to quantify the proportional contributions of fine sediment from hillslope surface and channel bank sources to suspended sediment and channel bed deposits before the impact of a forest wildfire in Na Borges, a Mediterranean groundwater-dominated river. It also compared burnt and unburnt spatial sources of sediment within a single catchment and the extent to which burnt material was transported downstream. The study focused on two small and steep sub-catchments, where just one of the catchments was partially affected by a wildfire. The pre-fire dynamics indicated that surface soils were the main sediment source in these ephemeral creeks. Post-fire sediment dynamics were characterised by a single flood event with a short recurrence interval (i.e. return period ca. <1 year). Sediment generated from the burnt area contributed 12% on average to bed-stored sediments within the burnt catchment, which reduced downstream to 5% along the main channel of the Na Borges River. The findings demonstrate the potential for using fallout radionuclide tracers to understand the wider impacts of wildfires on fluvial environments located outside of the burn area. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]