Suspended sediment transport diversity in river catchments following the 2014 phreatic eruption at Ontake Volcano, Japan

Abstract

Abstract The present contribution investigates the temporal changes in volcaniclastic sediment transport over the 2-year period after the 2014 eruption of Ontake Volcano in two small drainage basins where increased turbidity was observed immediately after the eruption. Two similar-sized catchments on the southern flank of the volcano, the Akagawa River (~ 4.4 km2) and the Shirakawa River (~ 2.9 km2) catchments, exhibited contrasting sediment delivery patterns and river water characteristics such as acidity and electric conductivity (EC). Increased turbidity, a high rate of suspended sediment supply, and elevated EC values were observed only in the Akagawa River, which hosts volcanic vents in its proximal part. The mineral assemblages and chemical characteristics of suspended sediment from the Akagawa River clearly indicate that the turbidity was derived from the erosion and reworking of primary eruptive material and lahar deposits. Previous airborne and remote surveys suggested the presence of primary ashfall and pyroclastic density currents in the upslope areas and valley heads of both the Akagawa and Shirakawa rivers. However, the river water characteristics and sediment transportation data of the present study clarify that the initial volcanic disturbance of the Shirakawa catchment was minor and limited. The influence of volcanic disturbance on the Akagawa River catchment continued for at least 10 months after the eruption and was also observed for an additional 9 months until the end of the snowmelt season in 2016. In the Akagawa River valley, two post-eruptive lahars that occurred during a 7-month period may have enhanced the removal of volcaniclastic deposits, and this remobilization may have resulted in diminished sediment delivery in the river after the lahar events. The results of this study provide information about the timing of the decline of suspended sediment delivery associated with small-scale eruptive activity, and such information may prove useful for evaluating the effects of other eruptions similar in size and character to the 2014 Ontake eruption. In addition, the approach adopted for monitoring rivers at downstream sites is clearly of utility for evaluating primary pyroclastic deposition and volcanic disturbance near inaccessible vent areas.

Keywords

• Ontake Volcano
• 2014 eruption
• Phreatic eruption
• Suspended sediment supply
• Volcanic disturbance
• Water characteristics