Historical Development of Landforms of the Todoroki Valley(Yazawa River), which Dissects the Musashino Terrace in Setagaya-ku, Tokyo

A Reconstruction of Knick Points Formed by Stream Piracy along the River

Abstract

The Todoroki Valley , which is famous as the only ravine in Tokyo wards, has been said that it had been formed by Stream Piracy which the Yazawa River deprived the flow path of the Kuhonbutsu River(Kaizuka, 1975). However detailed topographical survey has not been done in this area because of artificial river modifications of 1930’s which had extinguished a knick point(the Himeno Fall) located upstream of the piracy point.  Recently some historical records were found and have been introduced(Takasugi, 2011). The authors examined the documents and could restore a longitudinal profile of river bed of the Yazawa River prior to artificial modifications had been performed. Based on the profile, we clarified a historical development of landforms of the Yazawa River upstream portion from the Todoroki Valley to the Himeno Fall.