Characteristic Erosional Processes in Granitic Drainage Basins as Found in Tanakami Mountain Range, Shiga Prefecture, Japan

Abstract

The erosional processes in the stream channels and on the hillslopes are analyzed quantitatively in Tanakami mountain range on the basis of geology, geomorphology, hydrology and pedology of this region. As the dynamic balance of sediment does not hold on the stream bed, the rate of downward erosion of the channel is assumed to be a power function of the bottom shear stress due to the running water. Applying a Manning's type law of mean velocity to the hydraulics of the stream, the rate of the channel erosion and that of the channel retreat are expressed as some power functions of the stream discharge, the channel gradient, the channel width, and a coefficient representing the effect of such factors as geology and climate. The contemporary distribution of the knick points of varied "age" is related to the rate of their retreat. A statistical analysis leads to a concrete form of the relationship between the rate of the channel erosion and the above hydrogeomorphological factors. The hillslope processes are described as the processes conditioned by the channel erosion. The time series of developing profile of a hillslope is reconstructed from a sequence of hillslope profiles along the reach of a stream. It is shown that the hillslopes recede with the slope angle gradually decreasing when the rate of the channel erosion is small, and contrarily increasing when the latter is great, and that a parallel recession takes place as a long term average because the above processes take place in turn. It is also shown that three different weathering zones have respectively different fashions of denudation. The average and the maximum rates of denudation over this region under the natural conditions are estimated and compared with the actual rate under the bare-land conditions due to human activities. It is shown that the bare-land conditions in this region have accelerated the rate of denudation by a factor of more than ten. The above results are interpreted as being characteristic of granitic rocks being lifted under a lateral geomechanical pressure with fracturing, and being exposed to physical weathering.