Recession Rates of Knickpoints at Different Timescales in the Ata Ignimbrite, Satsuma Peninsula, Southwest Japan

Abstract

This study examined whether the rate of bedrock erosion was reduced with the decrease in precipitation during the last glacial period in the Satsuma Peninsula, southwest Japan, with special attention to the recession of knickpoints. A comparison of recession rates of knickpoints at different timescales was us as an effective method to assess this topic. Two suitable knickpoints were identified in the Nagasato River. These knickpoints along with four knickpoints located around the Kawanabe Basin were investigated. In this area, the Ata welded ignimbrite underlies the Ito ignimbrite. The restored depositional surface of the Ata ignimbrite, based on information from outcrops and borehole logs, strongly suggests that most of the knickpoints in the Kawanabe Basin originated immediately after the deposition of the Ata ignimbrite at 110ka. The recession rates of knickpoints since 110 ka were obtained numerically from the distances of recession between their positions after forming at 110ka and their current locations. On the other hand, knickpoint recession in the past several hundred years was derived from knickpoints formed by man-made cutoffs established to develop meandering riverbeds for new rice fields in the historical age. Thus, in the Nagasato River, the estimated average recession rate from 110 ka to the present is 2.0-2.6cm per year, and that of the last 315 years is 0.8-2.0cm per year. The recession rates of the remaining four knickpoints in the Kawanabe Basin were within several millimeters per year to several centimeters per year. These results all showed that the long-term recession rate during the period including the last glacial period slightly exceeded the recession rate of the much shorter period including only the postglacial period. The geomorphological evidence provided by the current research indicates that there was no remarkable reduction in the erosion rate throughout the past 110,000 years in the Satsuma Peninsula.