The dam operation took place in Luoyang late Tuesday night, as severe flooding devastated Zhengzhou, Henan province capital. Residents were trapped in the subwaysystem, and left stranded at their homes, offices, and schools.
Provincial officials announced that seven more people had been reported missing at a news conference.
Twitter user The Paper posted a video showing subway passengers standing in chest-high water muddy brown water while torrents raged outside.
The rain has caused disruptions in transport and work throughout the province. It turned streets into rivers that flow quickly, washing away cars, and rising into homes.
According to Caixin (a business news magazine), at least 10 trains carrying around 10,000 passengers were stopped, with three of them lasting more than 40 hours. According to the Transport Ministry's social media account, 26 highway sections were closed because of the rain.
According to the Communist Party committee of the city, a blackout caused the First Affiliated Hospital Zhengzhou University to shut down ventilators. Staff were forced to use hand-pumped bags to help patients in their breathing. According to the city's Communist Party committee, more than 600 patients were being transferred elsewhere.
According to the Henan Business Daily newspaper, a woman riding a subway through a tunnel filled with water told her husband that the water was almost reaching her neck and she had difficulty breathing.
It claimed that staff at a subway station had told her husband all passengers were evacuated, but he later acknowledged that it wasn't true after he began a video chat on his cellphone with his wife showing that she was still aboard.
Although the exact times and places of the deaths and disappearances were not immediately known, the province stated that more than 100,000 people had been evacuated to safety.
Henan province is home to many cultural attractions and is an important base for agriculture and industry. It is connected to multiple waterways. Many of these are linked to the Yellow River. This river has a long history, often burst its banks during heavy rainfall.
On Wednesday, the state media showed water at waist height with rain still falling.
The famed Shaolin Temple to the north of Zhengzhou was also badly damaged. It is well-known for its Buddhist monks' mastery in martial arts.
China experiences flooding every summer. However, the rise of cities and the conversion of farmland to subdivisions have made it more difficult.
U.N. Secretary General Antonio Guterres wrote a note to China's President Xi Jinping, "to express his deepest condolences on this tragic loss of life and devastation," Farhan Haq, U.N. deputy spokesperson, said Wednesday.