Abstract
The purpose of this study lies in analysing the development of the slope and fluvial processes in the area of the River Joganji, one of the typical torrential rivers in Japan. The upper part of this river, or the erosion caldera of the Tateyama Volcano, is very easily dissected and destructed because of the prevalent steep slopes and the presence of less resistant bedrocks containing solfataric clay. Enclosed by the calderawall lies a mass of mudflow derived from the caldera wall, and there extends a vast area of badland topography, which is the most important source of sediment down to the stream. (Fig. 1, a geological map of the upper part of the drainage basin.) In the lower part of the river there develops a large alluvial fan (Fig. 6).
The writer investigated the forming process of the river terraces in the upper reaches, and made clear that they are suitable as indicators to analyse the erosional development of the past. He also investigated the progress of the topogradhic change consulting with literary records of natural disasters written in old manuscripts.
The results are summarized as follows:
1) In the bottom of the valley develop river terraces extending about 10 km from the caldera wall of Tombi-kuzure to the lower reaches (Fig. 2). They were constructed chiefly by the mudflow deposits. (Cross sections of the terraces are shown in Fig. 3, and the longitudinal profile in Fig. 4)
2) The terraces are divided into two groups; the higher and the lower. The former, with the surface of deposited materials, is vestiges of the waste-filled valley floor which was constructed by mudflow. The surface of the latter was formed by continued downcutting accompanied by slight lateral erosion.
3) The mudflow was originated from the landslide which occurred on the slope of Tombi-kuzure, was large enough to notice the retreat of the part of the caldera wall. Judging from the literary records of the natural disasters, the large landslide and the mudflow occured in 1858 caused by an earthquake and snowmelting. The mass of the mudflow deposits is calculated roughly at 4.1×108 m3.
4) A close examination of the distribution of the mudflow deposits (Fig. 5, 6) indicates that the end of the mudflow in 1858 reached as far as the alluvial fan and the delta regions, about 40-50km down from the source. The mass, however, was deposited almost exclusively in the upper reaches, elevating the valley floor considerably.
5) The downcutting of the waste-filled valley floor in the upper reaches immediately after the mudflow deposition began at the steepest part of the profile of the river. The downcutting was faster in earlier stage. The relative height between the past valley floor and the present (illustrated in Fig. 7) suggests the amount and the mode of downcutting. The mass eroded during the past 100 years since 1858 is calculated to have been deposited covering the large part of the alluvial fan.
6) The severe erosion in the upper part of the rejuvenated valley might have deen caused chiefly by the steepness and the roughness of the waste-filled valley floor, and in the lower part, by the decrease in load.
7) In conclusion, it was clarified that the erosion in the River Joganji during the past 100 years has progressed rapidly enough to notice the conspicuous topographic change in the bottom of the valley and that this made it difficult to control the erosin in this river.
In the mountainous district in Japan, torrential rivers are usually found and the rapid progress of erosion such as in the case of the River Joganji must have played an important role in topographic change, particuarly in dissecting volcanoes and other slopes composed of shattered rocks.
