琵琶湖北岸，野坂山地における谷中分水界の形成過程

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タイトル別名
  • Process of divide in valley forming in the Nosaka Mountains, Southwest Japan.
  • ビワコ ホクガン ノサカ サンチ ニ オケル タニ チュウ ブンスイカイ ノ ケイセイ カテイ

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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.

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