Comparative study on the fluvial plain of the Shonai and Yahagi rivers, central Japan, based on the geomorphological land classification.

Abstract

The Shonai river plain is composed of small scale natural levee, upper delta, lower delta and natural levee of the Kiso river, and on the other hand, the Yahagi river plain is of fan-like natural levee, fan-like upper delta and fan-like lower delta. Morphometric data of the Yahagi river basin, i.e., altimetric frequency, dispersion of altitude and average of relief energy, are larger than those of the Shonai. Longitudinal gradient of the Shonai is gentle, only 5.3/1000, and that of the Yahagi is slightly steeper, 9.2/1000. The thickness of fluvial layer on the marine deposits accumulated during the Jomon transgression (6000-4000 yr BP) in the Shonai river plain is thinner than that in the Yahagi. The width of the flood plain is narrow in the upper part of the Shonai river plain because terraces are well developed on both sides of the plain. In the Yahagi river plain, the width of the flood plain is nearly same from upper to lower courses. The change of the Shonai river course has rarely occurred in the upper and middle parts of the plain, but frequently in the lower part. On the other hand, the change of the Yahagi river course has frequently occurred. -from English summary