Abstract
Field observations were carried out to clarify the structure of water quality in a reservoir with dicho-thermal stratification. Measurements were made for the vertical profiles of water temperature, dissolved oxygen, electric conductivity, iron and manganese in a reservoir of Shimajigawa-Dam in Yamaguchi Prefecture. In this reservoir, dicho-thermal stratification frequently occurred during the period from April 1987 to March 1990. The maximum thickness of the layer of dicho-thermal stratification reached about 40 m, and the maximum range of the temperature variation in this layer reached about 2 degrees. Thickness of the anoxic layer where the dissolved oxygen was lower than 10% of the saturate value ranged from 20 m to 40 m. When the dicho-thermal stratification occurs, vertical density structure should be estimated by considering the water temperature, salinity and pressure, i.e. depth. Manganese and iron showed high concentration in the layer of dicho-thermal stratification. This feature seems to reflect the biochemical action in this region. Results of water quality analyses imply that the behavior of the water in the dicho-thermal stratification influences the quality of whole water in a reservoir, and that manganese, iron and electric conductivity are useful indices of the behavior of the reservoir with dicho-thermal stratification.
