The concentrations of Cs-137 and Pb-210ex in fluvial sediment and soil were analyzed to determine the sources of fine sediment collected at 9 tributaries located in Nagasawa reservoir catchment, Shikoku Island, Japan. The major sedi-ment source was identified as subsoil derived from landslide scar or channel bank, but sediment production from forest floor was also identified from the existence of Cs-137 and Pb-210ex/Cs-137 ratio. It was indicated that sediment production caused by past landslide dominated in subcatchments with high percentage of landslide area, but sediment production from forest area was also estimated in subcatchments with small landslide area. The surface soil of land-slide scars were characterized by high concentration of Pb-210ex. The concentrations of Pb-210ex in sediment collected in Tebako-dani catchment increased with downstream distance. This implied that residence time of sediment could be estimated by comparison with direct deposition rate of Pb-210ex. It was concluded that the sources of fine sediment could be determined by analyzing the concentrations of Cs-137 and Pb-210ex in channel bed.
