第四紀後期における日本島河川の河床縦断面形の変化

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Longitudinal river profile is one of the most important parameters indicating river transportational processes. River longitudinal profile changes in response to climate and sea-level changes. Differences in the shape between Last Glacial River long-Profiles (LGRP) and Present River long-Profiles (PRP) reflect the change of the fluvial processes from the last glacial to the present. However, comprehensive geomorphological research on LGRP is still very few, although numerous studies have dealt with the shape of PRP. In order to clarify morphological characteristics of LGRP, 35 rivers in Japan were selected and their paleolongitudinal profiles formed during the last glacial period were reconstructed. Based on the comparison of the shape of the LGRP with PRP, this paper discussed how the downstream changes of the fluvial transportation processes are different between the last glacial and present. The curvature of river longitudinal profiles has increased from last glacial to the present, while the slope gradient of the river profiles has decreased based on regression analysis between distance from the present river mouth and altitude of both the last glacial and the present river beds. Excepting a few rivers strongly deformed by tectonic subsidence since the last glacial, LGRP have slope of over 1/1000 near the point below the present river mouth in contrast most of PRP less than 1/1000. Considering that the slope of the present fluvial fan front is about 1/1000, we can point out that at around the last glacial maximum period, gravels were transported to downstream at least around the present river mouth. Besides, the fact that slope of the LGRP slightly decreased downstream in contrast with that of PRP markedly decreases suggests that the last glacial rivers can transport coarse sediments more promptly than the present ones.
2011-07-25

第四紀後期における日本島河川の河床縦断面形の変化

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