抄録
The delta plains in Japan were reclaimed to a large extent since the end of the middle ages when the techniques for public works were remarkably improved in relation to the progress of fortification. In those days, reclama-tion work for a delta plain was generally carried forward from the inland part to the sea-shore. But in the case of the Kakogawa delta, Hyogo Prefe-cture, the portion on the left bank of the river (eastern half of the delta) was first reclaimed into paddy-fields in the Nara Period (ca. 700-800), but the western half was left unreclaimed, even at the end of the middle ages, left as a waste land devastated by the frequent shifting of the river course and the repeased invasion of the sea. Thus, the Kakogawa delta plain gives an exceptional example as to the process of reclamation of a delta plain. The reason why the estern half of the delta plain was at first reclaimed is supposed to be explained as follows. The Kakogawa delta plain is situated in the west of the Rokko block mountain which recently uplifted. So the eastern half of delta, lying nearer to the mountain was much more affected by the block movement than the western half was, that is to say, the delta was tilted from the east to the west. As a result, the main course of the Kakogawa was probably made to shift gradually from the east to. a distributary in the west. But the existence of the island-like submerged mountains, made up of quartz trachyte, dominating above the western half of the delta plain, which had acted as the obstacle to the flow of the western distributaries, now made the reinforced' stream run more wildly than before. Consequently, the recla-mation work and the river improvement in the western half of the Kakogawa delta became more difficult than before. Thus the western half was left unre-claimed even after the eastern half had been brought under cultivation. Yet, since the end of the middle ages, what with the development of pulbic works techniques and what with the good administation of the successive lords of Himeji, improvement of the river courses, construction of the sea dyke were carried out, and reclamation work was remarkably progressed in a short period.
