The dam operation was carried out late Tuesday night in the city of Luoyang, just as severe flooding overwhelmed the Henan provincial capital of Zhengzhou, trapping residents in the subway system and stranding them at schools, apartments and offices.
Another seven people were reported missing, provincial officials said at a news conference.
Footage posted on Twitter by news site The Paper showed subway passengers standing in chest-high muddy brown water as torrents raged in the tunnel outside.
Transport and working life have been disrupted throughout the province, with torrents of rain turning streets into rapidly flowing rivers, washing away cars and rising into people’s homes.
More than 100,000 people have been evacuated to safety. Henan province is home to many cultural sites and a major base for industry and agriculture.
State media on Wednesday showed waters at waist height, with rain still coming down.
To the north of Zhengzhou, the famed Shaolin Temple, known for its Buddhist monks’ mastery of martial arts, was also badly hit.
China routinely experiences floods during the summer, but the growth of cities and conversion of farmland into subdivisions has worsened the impact of such events.