荒川低地へ向かった後期完新世の利根川旧流路 : ―妻沼低地における旧流路の復元―

概要

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Many Rivers are concentrated in the central part of the Kanto Plain, central Japan. The Tone River, the largest river, flowed south-southeast before it was artificially diverted eastward at the end of the 16th century. A large meandering paleochannel in the present Arakawa Lowland, which is situated about 30 km south of the present Tone Rivers course, is thought to have been formed by the former Tone River. However, the upriver part of this paleochannel is indistinct in the Menuma Lowland lying north of the Arakawa Basin. Detailed aerial photo interpretation and observations of sediment samples obtained with hand augers reveal that a paleochannel with a north-south trend in the eastern Menuma Lowland contains volcaniclastic sediments derived from the Tone River catchment. The age of the buried paleochannel, i.e., the former Tone River, was estimated to be around 1,300 years ago based on many archeological survey reports of ancient tombs and other sites located around the paleochannel. The subsequent eastward natural diversion of the Tone River into the Kazo Lowland seems to have been caused by aggradation due to a rapid supply of volcanic sediments and tectonic subsidence in the Kazo Lowland situated in the central part of the Kanto Tectonic Basin.