抄録
In the Zaô valcano, the crater lake “Okama” has been famous as an active center in the historic time. Soon after the cease of the last activity in 1939 at the entrance of the Jigoku-sawa, several fumaroles were suddenly opened, in which the steady state has since been maintained for thirty years. In the early spring of 1966, the activity suddenly, occurred in the vicinity of Kamoshika Spa. Especially, near the junction of the Nigori-gawa and the Furiko-sawa, a new hot-spring started to gush out. Supplied with the acid water of the spring, the pH value of the river water became much lower than ever in its middle and lower course. Therefore, since 1966 the river water has not been used for the irrigation.
