Sanmenxia Reservoir is located on the middle reaches of the Yellow River, near Sanmenxia Gorge on the border between Shanxi and Henan provinces.
According to Chinese media reports, the accident occurred near a beach that was a popular photo spot on social media.
Red Star News reported the water at the spot suddenly surged, and people in the river were swept away.
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A survivor named Qin Jialei told Red Star News that he walked to the middle of the river, about 1.5km (0.9 miles) downstream from the dam, at around 4.10pm on Sunday.
Five minutes later, the river started rising, surging around 1.8 metres in just six minutes until Qin was completely submerged, according to the report. Qin said he then swam to a beach around 50 metres (164 feet) away.
Videos on social media showed people trapped in the water.
A staff member of the Sanmenxia Water Conservancy Project Administration denied the change in water levels was caused by water release from the reservoir, saying they had followed regular dispatch and operation instructions from a higher-ranking department.
The report also said fences along the river had been destroyed and there was a sign near the site warning people not to go into the water.
