CHANGES OF DEBRIS SUPPLIES AND CHANNEL FLUCTUATION PRESUMED BY STATISTICAL ANALYSES OF ALLUVIAL FAN FEATURES AND STRUCTURES

Abstract

The distribution of the periglacial area is limited to high mountain areas in Japan at present. But the periglacial areas spread over Hokkaido and mountain areas of central and northeastern part of Japan during the last glacial age. To the contrary, the most part of southwestern Japan were occupied by non-periglacial areas. The ascending of river bed during the glacial age and the descending during the post-glacial age have been pointed out in many upper reaches of rivers in Japan. But it has not been examined precisely whether the amounts of debris supplies and the fluctuations of river bed were different between the regions occupied by the periglacial areas and the non-periglacial ones. Therefore, this paper discusses these subjects.
First of all, the degrees of influences of factors such as the drainage basin area and the relief ratio on the alluvial fan features and structures represented by the variables such as the fan area and the thickness of fan deposit were assessed by factor anlysis (Fig. 2). As the values of factors are given, standard values of the variables are determined for the fan formed corresponding to the factors on the basis of the degree of influence, i. e., factor loading (Fig. 3). The tendencies of the change in debris supplies and of channel fluctuations were revealed by comparing the measured values with the standard values for each period classified as follows:
In the Pacific side of Japan, sediment yields were abundant in the Nakadai Stage (30, 000_??_50, 000 y. B. P.) and the rapid downcutting of river bed proceeded after the stage (Fig. 10). The amounts of debris were small in supply during both the Tachikawa Stage (10, 000_??_30, 000 y. B. P.) and the Holocene. Changes of sediment yields and channel fluctuations in the Coast of the Sea of Japan are in good agreement with those in the Pacific side, whereas channels in the former region have been more stable than those in the latter one. in Hokkaido, the river beds ascended during the period from the Shimosueyoshi Stage (90, 000_??_130, 000 y. B. P.) to the Nakadai Stage. But the small productions of sediments were conspicuous through the Nakadai Stage, the Tachikawa Stage and the Holocene. In the southwestern part of Japan, debris supplies were compara-tively abundant in both stages of the Shimosueyoshi and the Holocene, and channels have been markedly stable since the Shimosueyoshi Stage. The characteristics of sediment yields and channel fluctuations were shown in Fig. 11.
These phenomena for debris supplis and channel fluctuations can be explained by the following hypotheses: It was very cold in the Nakadai and Tachikawa Stage, and there was the smallest quantity of precipitation in the Tachikawa Stage (Fig. 11). A great deal of debris was produced in the source area of the mountain where the periglacial areas spread widely and there was the moderate amount of precipitation, and following it, debris yield was active in the source area where the non-periglacial zone prevailed and there was the large amount of precipitation (Table 1). When the circumstance changed from the non-periglacial condition to the periglacial one and the precipitation decreased, the river bed ascended, whereas, in the opposite change in condition, descending of the channel appeared (Table 2).
Consequently, the amounts of debris supplies in the periglacial areas are considered to have been greater than those in the non-periglacial ones. The amplitude of the channel fluctuation was smallest in the regions where the circumstance did not change from the periglacial condition to the non-periglacial one.