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International audienc

Purpose Wildfires represent one of the major natural disturbances within forested landscapes and have potential implications for the quality and function of downstream aquatic ecosystems. This study aimed to determine if a wildfire in a mountainous, forested watershed in British Columbia, Canada, caused a change in the dominant sediment source in the immediate 1-2 years following the wildfire, and if the sediment sources changed over the medium term (3-7 years) as the landscape recovered. Materials and methods Source materials (surface soil, subsurface soil and channel bank material) and fluvial (suspended and channel bed) sediment samples were collected over the period 2004 to 2010 from a watershed burnt by a wildfire in 2003, and from an adjacent watershed that was not impacted (FRNs) caesium-137 ( 137 Cs) and unsupported lead-210 ( 210 Pb un ). An unmixing model was used to calculate the relative source contributions of the fluvial sediment samples. Results and discussion 137 Cs and 210 Pb un were concentrated in the upper layers of surface soils in both watersheds and were statistically different to concentrations in subsurface and channel bank material. In the burnt watershed, FRN concentrations were greatest in the ash layer. Sediment sources as determined by the unmixing model were 100% subsurface/channel bank material in the unburnt watershed, while in the burnt watershed 8.5±2.5% was derived from surface soils. In both watersheds, there were no major changes in the relative contributions from surface soil and from subsurface/channel bank material over the period 2004 to 2010. Thus, while the wildfire did cause a change in sediment sources, it was fairly subtle and did not conform to the effects following wildfire described for other studies in contrasting environments, which typically document a major increase in hillslope contributions relative to channel bank sources. Conclusions There was a limited response in terms of finegrained sediment sources (and also sediment fluxes) in the burnt watershed. The reason for this muted response to a severe wildfire is likely to be the lack of precipitation, especially winter precipitation and the associated snowmelt, in the first year following the wildfire. Thus while the landscape was primed for erosion and sediment transport, the lack of a driving force meant that there was a limited immediate postfire sediment response.Keywords Caesium-137 . Wildfire . Disturbance response . Fallout radionuclides . Sediment sources . Unsupported lead-210

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