2007 Volume 68 Issue 1 Pages 15-27
Continuous spatial observations of the water quality of Lake Biwa and the Yasu River were conducted during 1997 and 1999. The water quality of the river shows typical seasonal variations, though with sudden increases in turbidity and decreases in both temperature and electric conductivity after a heavy rainfall. A comparison of water temperature between the lake and river indicates a seasonal change of river water dispersion in the lake. The river water flows into the lake surface layer in spring, intrudes into the thermocline in summer and fall, and flows into the bottom layer of the lake in winter. Based on these results, the renewal time of lake water is estimated to be 17.03% a year. This value is the result of a poor renewal ratio in spring and an acceleration of water exchange in summer and fall. The intrusion of a considerable amount of river water into the thermocline caused by a typhoon is especially important for the rapid renewal of lake water.
