|The aim of this study is to determine the relationship between the variation in copper and arsenic concentrations and the rurbidity in water samples collected from 1970 to 2010 at Akaiwa along Watarase River, including data from 1967 to 1969 for arsenic. The relationship between copper concentration and turbidity and that between arsenic concentration and turbidity were evaluated by testing the significance of the correlation coefficients obtained from their approximations of copper and arsenic concentrations in the river water samples for 40 years were related to the turbidity of the water. However, the reductions of the concentrations were somewhat different between copper and arsenic. Copper concentration had significantly reduced since Kusaki dam began storing water in 1976. In contrast, arsenic concentration had reduced before this event. From 1980 to 2010, the average copper and arsenic concentrations stable at values <0.014mg/L and <0.004 mg/L, respectively , and the turbidity has been less than 17. However, events of high turbidities and high copper and and arsenic concentrations have occurred irregularly in several years, especially in 1990. These events had been affected by discharge of water from Kusaki-dam after the typhoons that stuck in August and/or September in the corresponding years.