Hundreds of people had gathered to watch the natural phenomenon.
But the biggest return tide in nine years destroyed the barriers on the Laoyancang dam of the Qiantang River, east China's Zhejiang Province, injuring more than 20 tourists and security guards.
Having no time to escape, many tourists were washed down from an embankment about eight metres high.
A television presenter was also caught out by the massive wave (see video below).
This year the spectacular tide came earlier because of the influence of the typhoon Nanmadol, which hit Fujian Province, while packing strong winds.