琵琶湖北岸, 野坂山地における谷中分水界の形成過程 Process of divide in valley forming in the Nosaka Mountains, Southwest Japan
琵琶湖北岸, 野坂山地の谷中分水界2例を対象に, その形成時期および過程に関して地形発達史の観点から検討した。<br>国境における五位川流域のL1面構成層下部層は, 現在の流域に存在しない丹波層群起源の巨礫を含んでいる。丹波層群の分布は現在の知内川最上流部に限られている。したがって, かつて知内川最上流部は五位川へと流れ, 日本海へ向けて北流していたことを示す。<br>国境の谷中分水界は, 最終氷期後半に生じた河川争奪に伴う流路変更により形成されたものである。すなわち, 知内川最上流部から現在の知内川下流へ南流する現河道が固定化したのは, 2.2～2.5万年前 (AT降下期) 前後の河床高度の上昇に伴うものであり, このことはこの時期における河川の掃流力の低下が背景にあったことが指摘できる。<br>新道野の谷中分水界については, その形成時期は最終氷期以前に遡ることが明らかとなり, 国境付近の谷中分水界とは異なる結果となった。
The purpose of this paper is to discuss a process of channel diversion and the age of divide in valley forming by studying fluvial landform units.<br>Two divides in valley described in this paper are situated at Kunizakai and Shindohno in the Nosaka Mountains, Southwest Japan. River terraces in the study area are divided into three surfaces. H surface group including plural surfaces formed before the Last Glacial stage. L1 surface and L2 surface formed in the Last Glacial stage, the former is distributed widely and the latter is preserved in small patches. L1 terrace deposits are divided into the lower part which is composed of gravel, and the upper part is done of silty sand including AT tephra (22-25kyBP).<br>At Kunizakai divide in valley, R. Goi and R. Chinai face each other, the former flows towards the Japan Sea and the latter towards Lake Biwa. At the upstream of R. Goi, the lower part of L1 terrace deposits includes boulders of the Tanba Group which is not existed in the present basin. It shows the boulders are transported from the upper course of R. Chinai in which the Tanba Group is distributed. This is the evidence that the water of upper course of R. Chinai had flowed into R. Goi through Kunizakai.<br>At deposition of the upper part of L1 terrace deposits, the water at Kunizakai flowed southwards into the lower course of R. Chinai because the drainage became poor and valley filling progressed. Kunizakai divide in valley formed through this process and its formation age is during the accumulation of the upper part of L1 terrace deposits.<br>At Shindohno divide in valley, R. Asoh and R. Oh face each other. The former flows towards the Japan Sea and the latter towards Lake Biwa. There is no evidence that the channel diversion occurred in the geomorphic feature, the profiles and the gravel composition of L1 and L2 terraces. These indicate that Shindohno divide in valley formed before the Last glacial stage.
- 季刊地理学 = Quarterly journal of geography
季刊地理学 = Quarterly journal of geography 51(3), 179-187, 1999-09-01
The Tohoku Geographical Association