Contribution of the Maebashi Mud flow event to the fluvial landform development neighboring the confluence of the Tone River and the Agatsuma River, Central Japan

Abstract

The large-scale sediment supply from Quaternary volcanoes represented by volcanic debris avalanche or volcanic mud flow has great potential to change the landform of the downstream part of a drainage basin. Based on a detailed field survey, the present study discusses the long-term impacts of the "Maebashi Mud flow" event on the fluvial landform development of the region where the Agatsuma River meets its main stream, the Tone River, Central Japan. The relationship between landform development and the impacts of the Maebashi Mud flow event has not been examined in previous studies although the terrace landform is well developed. The Maebashi Mud flow is considered to have derived from the "Ohkuwa Debris Avalanche" that occurred at the Asama volcano around 24,000 years ago and to have flowed down the Agatsuma River channel as a corridor. In the study area, the terrace development was associated with the climatic change until the deposition of the Maebashi Mud flow. It is suggested that the Maebashi Mud flow destroyed and covered a part of the existing fluvial depositional terrace which had been formed during the interstadial of the Last Glacial period. It appears to have been positioned in front of the estimated flow direction of the Maebashi Mud flow. Then, the subsurface deposits like loess and soil which had been formed on it were thoroughly eroded and taken in to the Maebashi Mud flow itself by the event. The Maebashi Mud flow depositional surface on the existing fluvial terrace has a slightly gentler slope than the overlapping fluvial surface and has been preserved without marked modification. On the other hand, the mud flow sediment deposited within the valley was rapidly eroded by the fluvial process after the deposition. Then, the eroded volcanic sediment was redeposited in the downstream part of the study area, causing valley filling around the Last Glacial Maximum period related to the excessively decreased tractive force. This phenomenon is interpreted as the indirect influence of the Maebashi Mud flow event followed by the lag. The fluctuations in the riverbed were gradually regulated by the global climatic change again in the period of the termination of the Last Glacial period.