Geomorphology and Subsurface Ceology of the Alluvial Plain of the lower Yahagi River, Central Japan

Abstract

No all-inclusive reports have been made on the morphology of the alluvial plain and the alluvial fills of the lower Yahagi River. The authors investigated them through aerial photographic surveys and analysis of many boring data.
This alluvial plain morphology is characterized by the presence of numerous small microuplands all over the plain (Fig. 1). Although the area corresponds with the natural levee-back swamp type of the ordinary rivers, the channel pattern at present shows the braided one of bed morphology. The authors recognized it has a connection with the geology of the drainage area. A larger part of the drainage basin of the Yahagi consists of granitic rocks, so in spite of the natural levee-back swamp type, the braided pattern characteristic of the river transporting gravels is predominant near as far as the mouth of the River (Fig. 14). Numberless small micro-uplands are due to the micro-uplands in the braided channels, moreover they are supposed to have been made into “shima-batake” (micro butte-like uplands) by artificial changes of prolonged paddy field cultivation.
Whereas the sedimentary structure of this alluvial fills at the coastal area represents a well stratified slaty one (Fig. 5, 6, 7), the structure at the inland area north of the New Tokaido Line has a disordered and confusive one (Fig. 8, 9, 10). The authors concluded that these depend upon the expanse of the transported bed loads by tidal or off-shore currents in the open sea and the lens-like deposition of coarse materials in the channel belts on the alluvial upland or in the closed sea.
Using measured ¹⁴C age of these layers at the coastal area, the lower sands (LS) accumulated from about 10, 000 years B. P., and the middle muds (MM) about 7, 000 years B. P. and the upper sands (US) about 4, 500 years B. P. (Fig. 15). The coast of the maximum “Jomon” transgression may have lain a little north of the New Tokaido Line. By rapid and succesive alluviations, the coast at the age of late “Yayoi” may have lain near at the line which links Isshiki with Kira.
Wider buried terrace exists beneeth the Okazaki district (Fig. 7, 8, 9, 11), and the authors named it the Okazaki buried terrace (Fig. 4). This corresponds with the Koshido terrace on the upper Yahagi River. But they are discontinuous on the projected longitudinal profile (Fig. 13). The authors explained it by the presence of the conspicious gorge between Okazaki and Toyota, as the longitudinal profiles on the alluvial plain and buried valley reveal two graided profiles at the gorge as well as those of the Koshido-Okazaki buried terraces.