Fingerprinting Suspended Sediment Sources in the Nukabira River, Northern Japan

摘要

Geological setting is crucial to the occurrence of slope failure and landslides, resulting in prolonged suspended sediment runoff; in this sense, local geology can affect the magnitude of suspended sediment yield. To clarify the suspended sediment sources in a watershed with various geological units, hydrological monitoring and fingerprinting techniques using natural radionuclides were conducted during a heavy rainfall event in August 2010 in the Nukabira River watershed, a tributary of the Saru River, in northern Japan. A GIS analysis for slope failure and landslide areas was also conducted to investigate the distribution of potential suspended sediment sources. During the rainfall event, the dominant sources of suspended sediment were found to be areas consisting of metamorphic rock (31%), sedimentary rock (30%), and accretionary sedimentary rock (24%). The highest sediment yield was found in metamorphic rock, which is consistent with the dense distribution of slope failure and landslides identified by the GIS analysis. Active landslides can introduce a significant amount of sediment from the hillslope to the stream channel, providing a source of fine sediment. In the area of the accretionary basalt block, there was an inconsistency between the sediment yield and the density of slope failure and landslides, perhaps attributable to the lack of a fine particle fraction in the sediment deposited on the bare slope along the stream channel and riverbed due to the resistance of bed rock to slaking and weathering. These results indicate that natural radionuclides can be used in order to elucidate the suspended sediment sources and sediment yield of various geological units within a watershed.