Growth of Alluvial Plain and Land Development in the Lower Basin of the Naka River

Abstract

From the point of view that the recent alluvial plains are formed by running water and man's activities, the writer made a thorough investigation of the Naka River's deltaic plain in order to survey the growth of plain and the development of the plain.The result can be summarized as follows:1) Plain of the lower Naka has 10 topographical features. The fan surface is composed within the sphere of a circular arc (semidiameter: 6km) traced with Kami Ono as the vertex, and delta higher and delta lower lie in the margin. The former occupies the central part of plain, the latter is developed mostly on the delta side, excepting some lines radiately stretching in the central plain. Beach ridge and sand shore are largely growing at the advancing shores, but in the retreating shores they could not be found.2) At the time of the last glacial age, this region was covered all over with depesits of coarse gravels and formed a fan land which had moresteep slope than the surface of present land; later, clayey soil beds (about 10m deep) were deposited as the result of submergence. The basis of plain was gradually formed with the submergence after the middle part of Jomon Age. There were two old distributary channels, at first, flowing from the hills towards the north-east by east and the south-east by east. Therefore, alluvial action in this region had its origin in depositing on the delta side. With natural agency and artificial activities, after that, these channels changed by slow degrees their courses from west to east. Then the deposition in the central part of plain began. The number of distributary channels has become larger on the north side of the present course. This may be attributed it diastrophic movements in the direction of SW-NE. The changes of shore line are conspicuous on the north-side of the mouths of principal dis tributaries. Though the shore line that links river mouth and Wada Jima (lies to the north; not island) had taken a shape of convex about two handred years ago, it became to be in straight-line about 1870 and shows a concave shore line at present.3) In ancient times, the development of land reclamations on clearing was set forward around delta side, and in the Middle Ages it was gradually expanded into the central part of plain. The development of tidal land is characteristic of the Tokugawa-era, and nowadays the reclaiming work is kept on to make a new arable land at the rising part of delta.4) The land development in ancient times was selective, and in the Middle Ages the land utilization was made according with physical formation. The development in the Tokugawa-era, however, is getting to urge the slow physical development of plain. This artificial urging of development began from about 1750. And at present the inhabitants are reclaiming the foreshore from the sea.In short, the developments in ancient times and the Middle Ages were passive, leaving itself to circumstances; men were in the situation of being acted upon. In the Tokugawa-era and present times, on the contrary, challenges to the circumstances are being made actively.Not that the plains keep a record of the victory of man's activities over the physical history. In the Middle Ages, the land system of Jori type buried in the ground, and breaks of Bandai artificial levees and the other breakwaters shows a case of a temporary defeat of human history in the Tokugawa-era. Even now, shore lines are gradually receding because of erosion. The land depression attributable to earthquakes is increasing the damaged districts from sea-water.Concerning the study of recent alluvial plains, there is much that requires consideration from various angles in human activities.