Abstract
In warm-humid and tectonically active regions such as Japan it is important to investigate the geomorphic development of recent alluvial plains controlled by sea-level changes to prevent natural disasters. This is because human activities concentrate coastal alluvial plains composed of unconsolidated soft sediments (alluvium) including inner-bay mud, which amplify seismic intensity. However, we know little about the general shape of the alluvium quantitatively although several river basins have been investigated in detail.
We selected 33 trunk rivers throughout Japan and measured the Present River long-Profile (PRP) along with the Last Glacial River long-Profile (LGRP) defined by the depositional surface of basal gravel layers formed under the influence of falling sealevels during the last glacial period. The thickness of the alluvium given by the relative altitude between PRP and LGRP was basically in proportion to the size of the river basin, and it decreased linearly upstream with the exception of several rivers such as Shinano, Kiso, Oita, and Yoshino. Along these rivers, the alluvium is markedly thicker than expected probably because of tectonic subsidence occurring repetitively after formation of basal gravel layers. This implies that the fluvial response to sea-level change is sensitive enough to distinguish the effects of the accumulation of tectonic movements after deposition of basal gravels. In stable or slightly uplifted areas, the inland distribution of inner-bay mud is basically controlled by river basin size.
