The Yukawa River flows down through the region consisting of sulfur and pyrites which makes the waters acidic. Acidity of waters caused by oxidation of pyrites is taken place not only on the earth surface but also on the subsurface. The pH of waters are high in the snow season of winter and low in summer inversely the flow rates of river decrease. The oxidation of pyrites seems to be violent in summer it is caused not only by accelerated reaction at high temperature, but the concentrated high acidic waters retained in the interspaces among gravels.
