Studies on the Geomorphological Features of the Fluvial Plains in Japan Focusing the Distribution, Geomorphological Land Classification and Its Application

Abstract

(1) The geomorphological features of the fluvial plain are strongly influenced by the volume and quality of sediments transported from the upper reaches of the river. There are close relationships between the sand and gravels on one hand, and the landform of the upper basin on the other.
Generally speaking, the mountain region is characterized by its upheaval and the plains are depressed. The plains were formed by the deposition of sand and gravels which were transported by rivers from the mountains.
(2) Basically, the combination of the geomorphological elements of fluvial plain is:
Fan+Natural levee (back-marsh)+Delta.
The geomorphological elements were formed by the repetition of flooding.
One of the typical types of the plains is the Nobi Plain in the Central part of Japan.
(3) Distinct regional differences can be identified in forms of the combination of the geomorphological elements.
The river which has intermontane depressions and gorges with knick point in the upper reaches significantly contributes to the regional differences. A considerable part of the large size gravels transported from the upper reaches is deposited in the intermontane depressions, while only sand and small-sized gravels are allowed flow downward to the plain. Therefore, when the absolute volume of gravels was small, a small fan was constructed. This phenomenon has been clarified by several studies of river bed sediments in the Mogami River and other rivers. Whether or not the river flows in parallel with the island arc has a decided influence on these features.
The plains along the sea coast was influenced by the fluctuation of sea water level.
(4) The geomorphological elements such as fan, natural levee, back-marsh and delta, and their combinations show the history of flooding of the plain. Therefore, by making a geomorphological land classification map of the plain, one can predict the feature of flooding in the future. The accuracy of the geomorphological land classification map was proved by the Typhoon of Vera (so-called Ise Bay Typhoon) of 1959 at the Nobi Plain.