阿武川上流域の水系変化
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概要
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The Abu river with 83km in length and 593km^2 in drainage area has its drainage in the northwestern part of Yamaguchi prefecture and pours into the Ses of Japan in the City of Hagi. It is generally believed that volcanic activies and resultant changes of river system occurred in the upper part of this river. Except for M. Oga's study there is no detailed geomorphological study based on field survey of the process of these changes The authors conducted investigation of remnant geomorphic surfaces (mainly their altitudes and surface conditions) and their deposits, in order to understand the geomorphological history of this area. The process of changes of this river system may be sumarized as follows: 1) The upper branches upwards than the Deaibuchi depths of the Abu river was the upper ones of the Tsuwano river and flewn towards north-east. The upper streams upwards than Yokoyama of the Saba river jointed the Tsuwano river at Yoro. On the other hand, the Ikumo river, one of the bra nches of the Abu river, belonged to the drainage of the Abu river before. These phenomena are deduced by the altitude of the surface of basement complex. 2) Eruptions of the Miharayama lava dome group took place just on the river course of the Tsuwano. 3) There emerged former lake Tokusa dammed by the above volcanic bodies. The altitude of the lake level was about 320m. in height and the lake extended its area as far as the upper parts of the rivers Shinome and Saba. However, the basin of the Ikumo river was not covered with the lake. These occurences are deduced from the distribution of the lacustrine terraces and the above-mentioneed deposits. Emergence of the former lake is urgued by E.Takahashi to be dated in the lacustrine deposits. However, geomorphological evidences to fully support his thesis has not yet obtained by the present writers. 4) The lake water began to drain from the Deaibuchi depths of the Chomon gorge and river courses were converted from the former direction to present-day ones. On the other hand near Tokusa town-ship, there remained a small dammed-lake. by meanes of deltaic doposits of the branch 5) Following the subsidence, the lacustrine terraces were dissected or covered with talus deposits and the branches made fans on and/or blow the lacustrine terrace. In the basin of the Matsumoto river, the river meandered and formed terraces at several levels. At early tme of the subsiding the Saba river captiured.
- 1966-12-15