Hypoxia develops in bottom water in enclosed water bodies with limited water exchange and long water retention times, especially near the river mouth. The inner part of Tokyo Bay is an example. This study is focused on the development and decline of hypoxia around the Haneda Airport D-runway, which was constructed nearby the river mouth of the Tama River, and a comparison was made between monitoring data at a monitoring post at the Haneda Airport and those far off the river mouth nearby Kawasaki artificial island. Multiple sets of data have been incorporated, including water quality data collected every hour in monitoring posts by MLIT, and meteorological data of JMA. The hypoxia period and the average thickness of hypoxia were calculated, and the sudden recovery of DO was analyzed. The analysis revealed general characteristics of hypoxia development and its decline. Especially, three mechanisms of the DO recovery process were found to be dominated nearby the river mouth: strong and continued southerly wind, the intrusion of dense oceanic water, and extreme freshwater discharge of the Tama River associated with the typhoon. Year-to-year variations of hypoxia periods or volume at Haneda Airport showed no increasing tendency of hypoxia after the construction of the D-runway.