Landform development of mountains and rivers in Japan

Abstract

The geomorphic development of Japanese mountains and rivers during the Quaternary is the result of tectonic effects, sea-level change and climate change. The majority of the Japanese archipelago is being uplifted, with its mountains rising at rates ranging from 0.1 to 7 mm/y, as calculated by the mean altitudes of mountains under the assumption that they are in equilibrium between uplift and erosion. Filltop terraces built by climate-controlled accumulation of gravel are distributed throughout central and northern Japan. These terrace surfaces formed during glacial periods, and their surface ages are progressively younger upstream. This phenomenon was caused by sea-level lowering. These fluvial terraces are keys to reconstruction of geomorphic and topographic changes in Japanese mountain.Yonekura et al. classified the 61 mountainous areas of Japan into seven types, from the steepest type I to the gentlest type VII, on the basis of a steepness metric incorporating mountain heights, a steepness along the longitudinal and transverse profiles of ridges, and the heights of ridges relative to the valley floors. Half of Japanese mountains (types I to III) are in a state of equilibrium between uplift and erosion. Slowly rising or young mountains (types V to VII) have preserved uplifted peneplains on their tops and are not in the state of equilibrium.A proposed classification of river morphology into upper, middle and lower reaches, combined with the seven types classification of mountain morphology offers insight into the Quaternary development of Japanese landforms. The lower reaches of rivers in Japan are defined as lower area in elevation than the aggradation limit of sea level change. Terraces here are controlled by sea-level change. The middle reaches are defined as the part between point AL and the upstream limit of river terraces. Several terraces of older to younger ages are formed in middle reaches, indicating that the middle reaches are also uplifting. Sea level acts as the base level of erosion in the lower and middle reaches. River terraces are not formed in the upper reaches. Down-cutting for terrace emergence ceased at the LR, suggesting that the LR is stable. The upper reaches are defined by the LR point, which acts as a local base level of erosion.