Abstract
Continuous hourly measurements of the vertical distribution of water temperature and dissolved oxygen concentration in the impoundment of the Nagara Rivermouth Barrage showed that temporary thermal stratification developed under fine weather conditions with light wind and that it obstructed vertical mixing of the water, which in turn, might lead to an oxygen deficit in the bottom water. The temporary thermal stratification was maintained for a few days. Diurnal minimum oxygen concentration in the bottom water was recorded at twilight, and vertical mixing caused by cooling of the surface water at midnight recovered the oxygen deficit. However, complete mixing including the bottom water did not occur every day. Judging from the rapid oxygen consumption rate by the water column and sediment, the temporary thermal stratification could cause frequent oxygen deficit in the impoundment in summer.
