The water rose rapidly before it overtopped the Sanmenxia dam in Henan province, sweeping away people who had gathered below for photographs, the Sanmenxia Municipal Emergency Response Bureau said in a statement.
The bureau said the incident was under investigation and advised visitors to “pay attention to personal safety during the holiday period.”
Reports said water behind the dam was estimated to have risen by almost 2 meters (6 feet) within 10 minutes before it began spilling over the top.
The dam on China’s mighty Yellow River was completed in 1960 and has been troubled by sediment buildup. That has caused flooding upstream, leading to complaints about the dam’s design and management.
Chinese are expected to take more than 2 billion trips in the 40-day period surrounding Sunday’s holiday, which follows the lifting of rigid “zero-COVID” containment measures that had discouraged travel for most of the past three years.
It also comes a week after an avalanche buried vehicles outside a highway tunnel in Tibet, killing at least 28. Many of those trapped were headed home for the holiday.