ON THE MAKING OF PADDY-FIELDS AND IRRIGATION CANALS IN THE ELEVATED FANS ALONG RIVERS IN THE TOYAMA PLAIN

Abstract

The Toyama Plain is noted for its high degree of paddy-field making, that is, the percentage of paddy-field within the cultivated land, is the highest among all the plains in Japan. The Toyama Plain is mostly alluvial plain, but spacious elevated fans are found near the foot of the mountains. Paddy-fields have been developed over the whole district, even in these elevated fans, accounting for the high degree of paddy-field making in the Toyama Plain as a whole. Spacious elevated fans are developed in Shizuoka, Kanagawa, Saitama, Nagano and other prefectures, but many paddy-fields can hardly be found in the elevated fans of these prefectures. In general, the construction of irrigation canals (or reservoirs) is the basic element for the development of paddy-fields. The main purpose of this study is to make an inquiry into the factors that made the construction of irrigation canals possible in the elevated fans in the Toyama Plain, and also to throw some light on the kind of irrigation canals that have been built. The Toyama Plain is drained by rivers, those as the rivers of Oyabe, Sho, Ida, Jinzu, Joganji, Kamiichi, Hayatsuki, Katakai, and Kurobe, and two-terraced or three-terraced elevated fans are found at the mouths of the valleys of all these rivers, with an exception of the Sho River. The present writer has carried out a detailed investigation on the configuration of the ground in the drainage area, seasonal variations in the amount of river water, the development of irrigation works, the conditions of irrigation, etc., concerning each of these rivers. Moreover, he has drawn up nine maps showing the elevated fans and irrigation canals of each river. The results of this study are summarized as follows: It was in the Edo Period that, except for the highest part of the Tatenogahara elevated fan of the Oyabe River and a few others, and for the Osawano elevated fan, paddy-fields were developed in the elevated fans and irrigation canals were laid from the rivers. This is a surprising fact when we consider the history of irrigation in various districts of other prefectures in Japan. This is to be accounted for in part by the encouragements and assis-Lance given to the development of paddy-fields as a part of the agricultural policy of the Kaga Clan; but it is also partly attributable to the fact that there were rivers which made it possible, with the construction technique of the Edo Period, to bring irrigation canals in the highly elevated fans over comparatively short distances by utilizing the gradient of the valley bottom. Furthermore, the catchment area of the rivers between the mountains is not always wide enough to irrigate the paddy-fields located in the lower reaches of the rivers. However, since the district has heavy snowfall, during the period that requires the largest amount of water, that is, during the period of rice transplanting and its preparation, the rivers are supplied with abundant water from the melting snow, and there is always sufficient water available for the purpose of irrigation. However, as we have seen in the conditions of the elevated fans along the rivers of Oyabe, Joganji, Karniichi, Hayatsuki, Katakaki, as well as the elevated fans of Junikanno and Tanayamano along the Kurobe River, there is always shortage of water in the irrigation districts during the period from the end of July through August, and the disputes among irrigation unions were of frequent occurrence. It is for this reason that the strict rules of traditional use of water were observed. In recent years, as a result of the integration of irrigation canals and the construction of multipurpose dams, water use has been improved and progress has been noted for the methods of irrigation.