Dynamics of dissolved oxygen(DO) of the bottom layer in the lower reaches of a rivermouth barrage of the Nagara River was investigated. The tidal action is a primary component to govern the temporal variation of the daily-averaged DO concentration of the bottom layer of the Nagara River estuary and river flow gives disturbances to this primary mode.Such fundamental feature remains unchanged before and after the construction of the Nagara Barrage. When we have a big flood in summer season, DO concentration in the bottom layer of the estuary increases rapidly during a rising stage of the discharge, but after the peak discharge DO becomes very low because of the intrusion of anoxic saline water from theinner part of Ise Bay.
　The difference of DO variation in the estuary after the Nagara Barrage construction is caused by the difference in mixing regime associated with tidal action. The mixing regime in the spring tide changed from a well-mixed condition to a partially-mixed condition. Thus DO concentration of the bottom layer remains low even during the spring tide due to weaker vertical mixing with the Nagara Barrage in addition to the neap tide period.