GEOMORPHOLOGY OF THE REGION AT THE UPPER COURSE OF THE RIVER WATARASE

Abstract

The River Watarase is well known as a flood river in Japan, which causes great damages to the human life every year. The writers have studied on the geomorphology and geology in the upper region of the River Watarase, especially on valley forms, stream terraces, kinds of rocks and rock structures. Main subjects which are explained in this paper are summerized as follows: (1) The region at the upper course of the River Watarase is chiefly composed of Chichibu system (paleozoic rocks), granites, quartz-porphiries, diabases and other rocks. Landforms of this region are remarkably controlled by the distribution of these rocks and structures. (2) The writers have observed many fault plains in detail along the stream course of the Watarase, surveying dips and strikes of the fault plains, breccia, fault clay and shatter zones. It is concluded that many of the stream courses of Watarase and its tributaries are determined by the fault lines. Ages of those faults are old and many valleys of this region are faultline valleys. However, the writers have discovered an example of a new fault moved again after the building of terrace plains. In the region of Chichibu system, many trikes of faults are NNE-SSW or NE-SW and dips are almost vertical or high degree resembling the tendency of bedding plains of rocks, while in the region of granite, the strikes of the faults are generally NW-SE and the dips are of low degree. (3) The narrow stream terraces along the River Watarase are classified into three groups. Terrace plains are well developed in some local small basins formed at the cross points of fault lines. At the region of granite, Which forms a clear knick point of the river profile, terrace plains are not well developed, but many small cones are seen, formed by little tributaries from the mountain sides.