十和田シラスによる奥入瀬川水系の水質汚濁
1979 年 19 巻 4 号 p. 123-131
詳細
1979 年 19 巻 4 号 p. 123-131
The Towada Lake and the Oirase Valley are the famous picturesque places belonging to the Towada-Hachimantai National Park in Japan. The area along the Oirase and its tributary waters is covered with the so-called “Towada-Shirasu”, which consists of the Holocene tephras, non-welded pumice flow deposits, and tuffs.
The suspended loads in the Oirase and its tributary waters were the fine grained pumice, the silty to clayey volcanic ash, and the fragments of rotted leaves. They were responsible for the water turbidness of the Oirase River. The medium to coarse grained pumices were dominant in the bed loads. Both loads contained augite, hypersthene, magnetite, and small amounts of hornblende as heavy minerals. Montmorillonite was the dominant clay mineral in the suspended loads. These minerals were estimated to derive from the collapsed materials of the “Towada-Shirasu”, especially the Hachinohe Pumice Flow deposits and its underlying tuffs. The water turbidness of the Oirase River is directly concerned with the collapsed materials of the “Towada-Shirasu”.
In addition to the geological and geomorphological characteristics (Shirasu and the stage of youth), artificial factors such as a lumbering of trees and/or a grassland reclamation may promote the water turbidness of the Oirase River.
