Tran Viet, 21, of Mong Cai Town in Quang Ninh Province, was rowing a boat on the river when the tragedy occurred.

Besides heavy rains since late Saturday, unannounced dam discharge in China was also a cause of the floods, local residents said.

The flooding destroyed dozens of cargo boats in the river and submerged many streets in Mong Cai.

Hundreds of houses in the downtown area are under 30-50 centimeters (12-20 inches) of water and residents and authorities are grappling with the disaster.

Tuoi Tre newspaper quoted a top Mong Cai official also as saying China’s surprise dam discharge could be the reason for the heavy flooding.

Normally Chinese authorities inform Quang Ninh prior to discharging water into the Ka Long River, but it is possible that the heavy rains over the weekend quickly flooded the upper section of the river and did not give them enough time to make an announcement, he said.