Abstract
Through the ancient and the medieval times, the O-i River had reccurrently caused disastrous floods in its downstream lowland area, giving rise to changes of waterways, with the result that the Tokaido crossing the river and the post towns along it were subject to frequent vicissitudes. Scrutinizing many accounts of travels and ancient manuscripts, and examining microtopography with the aid of areal photographs, the writer traced back its vicissitudes.
During the Tokugawa Era, the Shogunate took advantage of the O-i River for the political purpose to maintain their rule, prohibiting the use of boats and the bridge construction, and retaining the practice of wading. This gave a great deal of troubles to the ordinary travellers and to the lives of people in the upstream mountain villages, but two villages on both sides of the river, or Shimada and Kanaya, were able to thrive as ferry settlements, especially when the ferry service had to be stopped by floods.
The people in the mountain villages sought for new traffic routes by land instead of by boat, which were to be routes leading from one drainage area to another through passes by use of many river systems or ridges on the remained peneplains. However, such policy of prohibiting the use of boats and the bridge constructiofi as was apparently against the public good had to meet strong resistance of people by use of their pleas.
In the Meiji Era, this political restraint was removed and transportation by boats began. The opening of the Tokaido railroad exerted a strong influence upon post towns along the Tokaido; some becoming prosperous and others deteriorated.
The recent development of science and technique has enabled us to subjugate the Nature; the O-i River was turned into a source region of abundant electric power by constructing a number of dams and power plants, which at once have set the people along it free from fear of flood damages and have provided the downstream area with sufficient irrigation water.
Not only between Shimada and Kanaya, but everywhere permanent bridges permitting trucks have been built. Thus the former divide-and-rule policy suddenly gave way to the harmonious rule. It is evidently demonstrated by the fact that annexation of towns and villages on both sides of the river have taken place by means of bridges
