Water quality of Japanese rivers and its variation duriag these several decades by the change of cultural, industrial and hydrological systems

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Based on the rivers studied by J. Kobayashi (1961), we surveyed water quality of 133 rivers from Hokkaido to Kagoshima areas. By comparing the results of Kobayashi and us, we discussed long term change of water quality of rivers from 1950s to present, regional characteristics of present water quality in respective rivers and the hydrological factors in the watershed affecting them. We concluded the followings based on such discussions. By anthropogenic input, Cl concentration increased heavily and Ca, Mg, Na and K concentrations inc eased slightly. The concentration of SO_4 also increased heavily by the effect from the combustion of basil fuel Increase of P concentration was observed but was not affected severely by human activities.In contrast to the above elements, Si concentration decreased in many rivers. We discussed the effects of various hydrological characteristics in their watersheds on Si decrease. The more the water-reserving capacity in the watershed is, the more the Si con … More centration decreased This result suggests that its decrease was greatly affected by the construction of water reservoirs in the watershed.We investigated the effect of a dam construction on the water quality of a river in Lake Okutsu which was recently constructed in northern part of Okayama Prefecture. We could not observe a remarkable decrease of Si concentration in the lake and its lower reaches, becalm this lake is existed at the most upper part of Yoshii River and was not biologically productive due to a low concentration of P in the river and lake waters These results indicate that accumulation of Si dissolved in the lake water into the bottom sediment in a dam is influenced by the input of P and N from its watershed and that its accumulation is possible to be highly active in a dam at the lower part of a river.We also investigated the dissolution of oxyanions, vanadate and phosphate, from lake sediments by the change of water quality in bottom waters. In both of natural and artificial lakes, their dissolution was greatly affected by the changes of water qualitiy in bottom waters and distribution process of chemical components at the water/sediment interface. Less