Abstract
The Natori River basin in northeastern Japan is one of the most suitable regions for understanding valley-filling in Japan. In this area, a filltop terrace with a steep longitudinal profile is known to be present. Although the distribution of this deposit is fragmentary, it offers a key to understanding thedynamic response of the total fluvial system to climate change since the Last Interglacial age. The purpose of this study is to clarify the relationship between the valley-filling and hillslope processes.
In the Natori River basin, fluvial terraces that were formed since the earilier phase of the Last Glacial age are classified into the following three surfaces, the L1, L2, and L3 surfaces, from older to younger. The L1 surface is a depositional terrace that has thick deposits more than 35 m in maximum thickness. The top of L1 deposits are covered or intercalated by the Aira-Tanzawa tephra which fell in 24 ka. The L2 and L3 surfaces are fill-strath terraces. From the viewpoint of chronology, the L2 surface was formed after the LGM. In addition, valley-filling began before the oxygen radio-isotope stage 4, suggested by radiocarbon dates collected from valley-fill deposit, and the longitudinal profile of buried valleys that were formed during stage 5. In short, valley-filling began before stage 4 and ceased at about the LGM in the upper part of the Natori River basin.
Along the Kita River, which is the main tributary of the Natori River, the L1 deposits contain numerous large irregular boulders and partly exhibits inverse grading. The features suggest that the L1 deposits in this area were transported by debris flow. In addition, valley-filling deposits consist of thick deposits of fluvial origin alternating with slope deposits in the upper reaches of the main stream of the Natori River basin. The slope deposit is composed of subangular gravel abraded by running water with sandy matrix. These facts strongly indicate that debris flow and the slope processes affected by running water were important for valley-filling in the Last Glacial age. In addition, footslope deposits indicate that debris transportation affected strongly by running water was inactive in the LGM. This period of disappearance of the slope process affected by running water corresponds well with the termination of valley-filling. The change in total fluvial processes, in which hillslope and stream processes are combined as above, corresponds well to the summer-monsoon oscillation as known from various analyses in deep-sea and lake deposit cores and aeolian deposit.
