Water levels on the upper parts the Yangtze are at their highest since 1987, the state-run Xinhua news agency reported.
Wen Jiabao, the Chinese prime minister, has ordered officials to prepare for "more serious floods and disasters".
On Saturday, Wen was touring flood damage in the central province of Hubei, where the rising waters have put pressure on the world's largest hydroelectric project, the Three Gorges Dam.
Hubei flood control officials told the Associated Press that the water level at the dam was 17 metres from its maximum capacity of 175 metres as of 8am (00:00 GMT) on Saturday.
China has for years promoted the Three Gorges Dam as the best way to end centuries of floods along the Yangtze basin and dismissed complaints about the enormous environmental impact of the $23bn reservoir that has displaced more than 1.4 million people.
Wen said the dam has played an important role in flood prevention along the Yangtze, Xinhua reported.
More than 1,000 people have died or disappeared in severe flooding in China so far this year, the highest death toll since 1998.
The flooding has also caused economic losses of at least $22bn and affected 120 million people, the government has said.
It has triggered fears of a repeat of disastrous Yangtze floods in 1998, the country's worst in recent memory, which killed more than 4,000 people and forced the evacuation of 18 million.