Abstract
Recent studies of river terraces in some mountainous regions in Japan revealed that the sequence of river terrace development during the late Quaternary was similar for adjacent rivers, showing that climatic changes had the same effect on all the adjacent drainages. However, landform processes in river basins are affected not only by climatic changes but also by the geomorphic and geologic conditions of the basins. Thus, landform development in a given drainage should vary corresponding to the geomorphic and/or geologic conditions subject to the same climatic changes. To verify this, the author examined the differences in landform development during the Late Glacial and the Post-Glacial Ages among eight drainages around the Matsumoto Basin (Fig. 1). The results are as follows:
1) Analyses of the areal distribution and interpolated profiles of fluvial surfaces (Figs. 1 and 3) suggest that the processes of fan formation are divided into three types. In the drainages of Chigawa(1), Ashimagawa(2) and Nakafusagawa(3), , alluvial fans aggraded during the Late Glacial and the Post-Glacial Ages, but the process is no longer ongoing at present. In the drainages of Karasugawa(4), Kurosawagawa(5) and Susukigawa(8), active aggradation of alluvial fans has continued since the Late Glacial. In the Kusarigawa(6) and Metobagawa(7) drainages, aggradation of alluvial fans during the Late Glacial and the Post-Glacial Ages was less active than in other drainages.
2) Mountain slopes in the drainages are divided into four units by three marked convex breaks in the slope (Fig. 2). The lower two units, i.e., higher and lower dissected slopes, were formed during the Late Glacial and the Post-Glacial Ages as a result of the effect of running water combined with the change in climate from cold-dry to warm-humid conditions. Based on the ratio of the area of the dissected slopes to the area of all mountain slopes (PD), the drainages are classified into three groups which are at different stage of dissection. In the Chigawa(1), Ashimagawa(2) and Nakafusagawa(3) drainages, the values of PD are large, while in the drainages of Kusarigawa(6) and Metobagawa(7), the values are small. The Karasugawa(4), Kurosawagawa(5) and Susukigawa(8) drainages have intermediate PD values (Fig. 5).
3) The types of alluvial fan development correspond to the different dissection stages of drainage. This fact indicates that the debris forming the alluvial fans was produced and transported through the development of the dissected slopes (Fig. 6).
4) Statistical analyses of the relationships between PD, dispersion of altitude (D) and geology give the following results. The value of PD increases with D (Fig. 7), showing that the dissected slopes were developed widely where D is large (Fig. 9). On the other hand, the value of PD varies with geology at the same value of D, though the value of PD increases with D for the same geology (Fig. 10). These relationships indicate that the large-scale relief and geology played independent parts in the development of the dissected slopes (Fig. 11).
5) The value of RPD, which is the residual after the generalized PD is subtracted from the observed one (Fig. 12), decreases with increase of RH (Fig. 15), which represents the generalized 'H (altitude) subtracted from the observed one (Figs. 13 and 14). This fact indicates that the value of PD increases as H decreases under the same relief and geological conditions, implying that the dissected slopes have developed gradually, from valley bottoms to the upper parts of valley side slopes. Thus, it is concluded that relief, geology, and altitude participated independently in the development of the dissected slopes.
The processes and development of landforms in the drainages are summarized as follows. With climatic change, the dissected slopes were formed on mountain slopes in all the drainages during the Late Glacial and the Post-Glacial Ages.
