The Sekigawa rises in the Myoko, Kurohime and Yakeyama volcanic region along the border-line between the prefectures of Niigata and Nagano in Central Japan. In its lower reaches, it flows through the Takata Plain. From the neighborhood of Takata City downwards, the river is called the Arakawa, until it empties into the Japan Sea.In the present paper, The writer discuss primarilly the development of irrigation in the region on the right bank of the Sekigawa. He will also make reference to the relation between the development of water-power generation and irrigation.
For the most part, the region on the right bank of the Sekigawa on the Takata Plain consists of fans and deltas formed by the tributaries, the Okuma, Bessho, Kushiike and Iida Rivers which join the Sekigawa on the Plain. However, the greater part of the irrigation water for the paddy fields in this region is supplied, either directly or indirectly, by the Uwae and Nakae irrigation canals which are connected with the mainstreams of the Sekigawa. Judging from the course of these two canals and the land surface form of this region, it may be conjectured that these two large canals were not built at the time when paddy fields first appeared in this region, but that canals built at an earlier date had been used to feed the paddy fields. Historical investigation, as a matter of fact, has established the correctness of this conjecture.
The next question that arises is this a Why did the old canals become useless and why did the two large canals have to be built? The catchment area of the tributaries in mountain districts is small, so that in the early days of the Edo Era (1603-1867), with the sudden increase of paddy fields in the alpine and mountain districts, the supply of water became insufficient for the fields located about the lower reaches of the river. As a result, the Nakae irrigation canal was first constructed. After the early days of the Edo Era were over, there was a slowing down in the increase of paddy fields in the plains, but the increase continued in the mountain districts. The reason for this is that the mountain districts of this region form one of the largest landslide areas of this country, noted for the remarkable development of the landscape of “rice terraces”. It was no easy task to make these rice terraces, so that it took a long time before any increase was noted in their number. Rice-growers here also tried their utmost to build irrigation canals or ditches and reservoirs. Hence, as paddy fields increased in the landslide area, the volume of water that flowed off therefrom diminished. Thus water shortage became gradually more acute in the upper reaches of the plains, leading to the extension and enlargement of the Uwae irrigation canal in order to feed this region.
Even with the construction of the Nakae and the Uwae irrigation canals, water shortage was always acute in the regions in the lower reaches of both canals during the period of water shortage in summer. Hence there were constant disputes over water rights between the Uwae irrigation district and the Nakae district during the water shortage period.
The development of water-power generation of the Sekigawa was begun in 1907. However, the water volume of the river showed a remarkable decrease due to snowfall in winter and to drought in summer, so that the electric power company could not obtain a constant of electric power. In order to remedy this drawback of the Sekigawa, the power company thought of a plan to utilize Lake Nojiri, one of the sources of water supply for the Sekigawa, as a reservoir. According to this plan, water was to be led into the Sekigawa from Lake Nojiri during the period of water shortage to increase its volume of water, and on the other hand, during the period of snow-melt or during the typhoon season, the surplus water of the Sekigawa is pumped up 85 meters into Lake Nojiri.

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In the Japanese Islands, geotectonic movements have been, generally speaking, very active since later Tertiary and the crustal movements which have immediate influences on present landforms are also supposed to have no small significance in geotectonic development. Accordingly, it is an extremely important problem in geomorphology to clarify the relation between geomorphic and geotectonic developments. From this point of view, the writer investigated geomorphic development of the Takanosu Basin, Akita Pref., northeast Japan, which was located in so-called Green Tuff Region, one of the Neogene orogenic zone in the Japanese Islands.
The results are summarized as follows:
1) In the first half of Pliocene, Japan Sea spread over the Takanosu destrict.
2) In the latter half of Pliocene, Nanakura mountains began to upheave, defining the western boundary of the Takanosu Basin, and Mitsukita formation, composed of lacustrine and coastal sediments, was deposited on its eastern foot.
3) At about the end of Pliocene, the crustal movement became more active. Nanakura mountains were elevated and the eastern boundary of the Takanosu Basin was defined by step faults. Thus the Takanosu Basin was formed and in accordance with the subsidence of the basin lacustrine Yuguruma formation was deposited in it.
4) With the retardation of the crustal movement, the basin was filled up with Yuguruma formation and an erosion surface was formed in the marginal zone of the basin by lateral erosion of rivers flowing into the basin.
5) After the formation of the erosion surface, the crustal movement became active again. The whole region was upheaved against the base level, being relatively warped down in the central part of the basin. The bottom of the basin was trenched by the Yoneshiro River and its tributaries and several levels of fluviatile terraces were made. It is inferred from the character of the terrace deposits and the distribution of the terrace surfaces that the crustal movement of the same tendency as what formed the Takanosu Basin was in action during and after the formation of the terrace surfaces.

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On June 28, 1961, a disastrous flood took place at various parts of the Tenryu River course caused by heavy rainfall accompanied by the Baiu front.
In the district of Kawaji, that is situated at the southern end of ma Basin occupying the upper part of the Tenryu River drainage, a violent inundation occurred and the deposits brought by the flood water covered the inundation area thickly.
Except those of branches, the flood deposits are divided into three categories, that is, gravelly, sandy and silty materials. In the cases of the sandy and silty materials, it is recognized that the deposits are well sorted. Namely, grain size distribution of most samples from these two is limited between the two sizes as is shown in . Fig. 4. Then, the writer determined graphically the finer limit of the sizes of a sample and presumed that the particles of sediments smaller than this size moved in the form of suspended load and flowed out with flowing water.
If we represent the value of this critical size by “d₀”, and the value of settling velocity of do in standing water by “w₀”, w₀ may be proportional to values of the vertical component of the turbulent flow which are functions of the velocity of the main flow.
The distribution of w₀ is shown in Fig. 5. Considering the foregoing aspects and Fig. 5, it may be interpreted that the flood-flows reduced abruptly its velocity near the entrance of the studied reach, and its high velocity zone forms a meander in the middle part of the flooded area while the flood deposits were being settled.

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In the northeastern Shikoku, the Asan Mountainland, whose southern boundary coincides with the Median Dislocation Line dividing southwestern Japan into the Outer and the Inner Geological Zones, is a recently elevated massif preserving several steps of relatively flat erosion surfaces on its foot.
The writer tried to make clear the Plio-Pleistocene tectonic movement of this mountainland through the study of these erosion surfaces. He classified these surfaces into 3 grouper, where sedimentary remnants are classfied into 6 layers on the northern side and 9 on the southern side of the massif. Moreover, 6 types of the conditions of contact between each surface and other ones are identified. Analysing these geomor-phological factors and fault structures, the writer reaches the following conclusions.
The Asan Mountainland is presumabjy a tilted block with some fresh fault scarps on its southern side as on the northwestern side, and has swollen up as a landwave of pitching with an axis upwarping at its central part linking Inohana with Sarusaka. Thus, the central part of the massif has been brought higher than its western or eastern ends. The movement mentioned above develops a system of radial faults with such depression areas as some parts of Takamatsu and Marugame Plain.
Considering the fresh fault cutting the lowest terrace gravels seen on the southern side of the mountain, this orogenic movement may have continued to grow up to the recent time.

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