Resumen de River terrace development by concurrent fluvial processes and climatic changes
Along the Usui River in central Japan, three well-developed terrace levels are observed. Each terrace is subdivided into two segments: a filltop terrace along the upper reach and a strath terrace along the lower one. Three valley fillings with coarse gravel beds in the upper reach correspond to the last three glacial ages. The positions of these valleysfills migrated upstream from the older terrace to the younger one.
To clarify the response of fluvial systems to climatic change, terrace sediments were interpreted from the viewpoint of paleohydraulics. Using the grain diameter of terrace sediments, paleo-tractive forces were evaluated based on the correlation between tractive force and grain diameter of the sediment on the present river bed. The valley filling was caused by the diminution in tractive force related to the decrease in heavy rain produced by typhoons during the glacial ages. During the interglacial ages, the reverse responses occurred due to the increased incidence of typhoons. From another aspect of the development of river landforms, the decrease in the liver channel gradient during the interglacial ages caused the transportational and/or depositional area of coarse gravels to migrate upstream in the subsequent glacial ages.
