GEOMORPHIC EVOLUTIONS OF THE ALLUVIAL LOWLANDS IN JAPAN

Abstract

Alluvial lowlands occupy about 13% of the total area of the Japanese Islands, and widely distribute along the coasts and the lower reaches of the rivers. They have been developed under the influence of environmental changes in the late Quaternary, especially under the eustatic change of sealevel. However, geomorphic evolution of each lowland is quite varied one another. In this report, the author tries to make clear the geomorphic development and regional difference of alluvial lowlands in Japan by means of the reconstruction of paleogeography. On the basis of paleogeographic development, alluvial lowlands in Japan are classified into six developmental series: F, N-S, D, D-S, V and C. The F series lowland is mainly composed of alluvial fan constructed since the Lastglacial age. Examples of this series are the lowlands of the Fuji, the Oi and the Kurobe Rivers. The N-S series takes its own evoultional course as follows: At the stage of the lowest sealevel in the Lastglacial age, the lowland had been composed of narrow alluvial fan or valley bed in the valley cutting the lower reach of the river by the lowering of sealevel. As a result of the transgression, the lower part of the lowland was drowned and changed to the lagoon in the Lateglacial age. During the former half of the Holocene, the lagoon was extended to the inland area, and the lower reaches of the river had changed into the flood plain composed of sandy and silty sediments. The lagoon had been reclaimed by sediments in the latter half of the Holocene. Then the landform of alluvial lowland was characterized by the alluvial fan, the flood plain and the sandy bank from upper to lower reaches of the river. Examples are lowlands of the Sagami and the Tenryu Rivers. The evolutional process of D series-alluvial lowland is as follows : At the stage of the lowest sealevel in the Lattglacial age, the lowland was composed of narrow alluvial fan in the valley developed between river terraces, and they were such landforms as the N-S series. In the Lateglacial age valleys were drowned from their lower reaches. In the former half of the Holocene, river terraces and valleys were filled with alluvial sediments, and the sea was widely invaded towards the upper reaches. Then the remarkable delta was constructed burying the shallow part of this inland bay. In the latter half of the Holocene, the delta has continued to develop seaward and floodplain deposit has also extended to the inland area. We can find good examples for this series in the evolutions of the Tokyo-shitamachi, the Tama River and the Yahagi River lowlands, and the Nobi plain. The D-S series is subdivided into (a) and (b) types. Both series are characterized by lagoon and sand bank in the Holocene. The lagoon had spread out since the Lateglacial age or the early Holocene accompanied with the sealevel rise. The lowland was mainly composed of both floodplain and delta facing on the lagoon, which was gradually reclaimed in the late Holocene and became swampy land. The topography of D-S (a) series lowland in the latest Pleistocene is the same with that of D series. During the Lateglacial age, the landform of D-S (b) series was composed of narrow floodplain and delta. Lowlands, such as the Kushiro, the Tsugaru and the Fukui plains belong to D-S (a) series. The Ishikari and the Kawachi plains can be classified into D-S (b) series. The evolutions of V series such as the Yamada, the Tsurumi and the Ota Rivers are characterized by the drowned valleys which have extended since the early period of their evolutions. The reclamation of drowned valleys had proceeded in the late Holocene. The lowlands of C series had been developed in the valleys during the Lateglacial age. The alluvial lowland was composed of either the narrow alluvial fan or the floodplain and delta at this time. The lowland became facing to the open bay by the sealevel rising in the Holocene.