Oyachi, situated in the northeastern corner of the Yonezawa Basin (Fig. 1), is a kind of bog lake with an area of 1020 ha, 950 ha of which is used as rice fields and the rest, 70 ha, is left as marsh. The rice fields here are classified into the two, the “Yachita” (marshy paddy fields) and the “Ukita” (floating paddy fields), the area is 700 ha. and 250 ha. respectively. The Yachita occupying the marginal part of Oyachi makes the muddy fields of powdered peat filled with water throughout the year. The Fkita which mainly occupies the central part is rather recently reclaimed fields. In Oyachi there run very many waterways and “Yachibune” (a kind of boat) is used to carry fertilizer or rice plants. When the uncultivated part or a ridge 4_??_6m. wide between rice fields is stamped violently, the ground roundabout shakes. The peat layer can be penetrated with boring stick by one hand, and as soon as it is pierced through, no resistance is felt. When the boring stick is drawn out, brown liquid mud blows out with methane for 5_??_7 seconds, then it rests at the depth of 3_??_5c.m. from the surface. The subsidence of the surface of Oyachi took place during 1931_??_1952 amounted 60_??_90cm. as shown in Fig. 2.
Having an idea that Oyachi is a kind of bog lake, the writer made surveys in summer, 1953, at 423 points in Oyachi area on (a) thickness of the peat layer, (b) depth of the water lying beneath the peat, (c) thickness of the mud covering the base and (d) depth of the base from the surface of Oyachi. The results are shown in Fig. 3_??_8.
(a) Although thickness of the peat layer (Fig. 3) ranges from 1m. to 2m., it is generally thick at the central part and thin at the marginal part of Oyachi. The peat deposits are saturated and underlain by water of low temperature, so that, for example, when the surface temperature of Ukita is 29.8°C, the temperature at a depth of 50cm. is only 1.4.9°C. Therefore, growth of rice plants is poor, and even alpine plants grow there, though the altitude of the area is only 212m. above sea-level. (b) Fig. 4 shows the depth from the surface of peat layer to that of the mud, namely the shape of the lake basin. The deepest point is 8m. in depth, and besides there are three sub-lake basins. The southern half of Uyachi has only a depth between 2 and 4m. Fig. 5 shows the depth of Lake Hakuryih, which i s situated in the deepest basin of the Oyachi bog lake. The maximum depth of water, however, is only 1.9m. That is to say, the bottom of the lake is a false bottom. (4) Fig. 6 shows the shape of base of the bog lake by contour lines in altitude above spa-level. As the result of borings, six rises and eight hollows were discovered in the base. The lowest (deepest) portion coincides with the area of Lake Hakuryu. Comparing Fig. 6 with Fig. 4, it is observed the location of the deepest zones of the base deviates to the east from those in the lake basin, and unevenness of the former is remarkable. (c) Fig. 7 shows the thickness of the mud deposited on the base and it amounts 8m. at the deepest part. It proves that the lower the base lies the more the mud has been deposited. Fig. 8 is the cross section along A-B (see Fig. 1) showing the relation between peat layer, water, mud and base.
It had been planned to reclaim Uyachi by draining so as to make the area into a land suitable for cultivation, and the Biruzawa Reservoir with a pondage of 2, 190, 000m3. has been completed. Now the draining project is just to be carried on. But as the quantity of water in Oyachi bog lake amounts 20, 000, 000m3., it is necessary to make a draining canal from a point 12km. distant or to drain the area by electric power. After draining was accomplished, the writer thinks, it is necessary to bring enough soil from other places into the area or to take off the peat deposits from the area.
