Mt. Gassan is one of the famous volcanoes of the Dewa Range. Around the volcano, there are four river systems, i. e. the Sagae-gawa, the Karasu-gawa (Dozangawa), the Tachiyazawa-gawa and the Aka-gawa. The present studies have been carried on since 1954 with the aim at elucidating the limnological features of the revers with special reference to the volcanic activity. The results of the investigations of the Karasugawa are, sumarised as follow:
1. This river is mainly contaminated with the water from Nagamatsu copper mine, the Nigamizugawa and Hijiori spa.
2. The chemical contents of the river water indicates the seasonal varia-tion, which is similar to that of the Akamatsugawa (one of the main bran-ches). Generally speaking, the concentration of the chemical constituents seems to be influenced by the amount of precipitation except during the snow-melting seasons.

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