SEDIMENTOLOGY OF THE MOUNTAIN RIVERS WITH MORE AND LESS CLASTIC MATERIALS IN THE JAPANESE ISLANDS

The sedimentology of the three rivers, having characteristic drainage basins, in the Japanese Islands is described and classified in the viewpoints of longitudinal profile, transportation, quantity of clastic materials and the stage of mountains. Simplified characters of the three rivers are as followed; the Abe River, large amount of supply of clastic materials at the early stage of mountains; the Kuji River, gentle supply of clastic ones; the Sato River, the least clastic supply among the three rivers at the late stage. The Abe River with the typical profile like the curve of the exponential function has very broad canyons with a lot of clastic materials. In the drainage basin of the Abe, the mountains composed of Tertiary rocks supply more clastic materials by big landslides. The mountains are uplifting with the highest speed of 40 cm / 100 yrs (4 mm/yr) in the Japanese Islands. The Kuji River in Abukuma Mountains has less clsastic materials than the Abe River. The channel of the river is controlled by Tanakura tectonic line. This river with many big boulders forms often riffle and pool on the river bed. These boulders have smooth, polished, and clean surfaces at the upper stream side. In contrast, the downstream surfaces of these boulders are rough, irregular and dirty. It seems that these boulders could not be transported for the long time, more than tens years. We have been watching some big boulders at the point fixed from April in 2005. As the result, no big boulders were transported with the survey through photos, even if the high water on this June was recorded at two meters higher than the ordinary water level. The Sato River, which is one of the tributaries of the Kuji, has roughly a similar profile like the curve of the exponential function except a convex curve at the middle part. This river, which is mainly occupied by granite and gneiss with the minor amount of Miocene conglomerates in the drainage basin, has less clastic materials than the Kuji River, because the basement is often exposed along the river bed. Many big boulders (ÅÑ1 m) are trapped at the river bed. These boulders were supplied to the river by the tectonic movement at the early stage of the river, and are reworked now.