China says flood waters at the Three Gorges Dam will peak within the next 24 hours, after torrential rain further up the Yangtze river over the weekend.
More heavy rain is expected in parts of southern China from now until Thursday.
News has only just emerged of a bridge collapse in Henan province on Saturday in which 33 people died and up to 21 are still missing.
China suffers monsoon-type rains every year but this year's rainfall has been the heaviest in more than a decade.
Water levels in one tributary of the Yangtze river were reported to be the highest in 30 years this weekend.
Since Sunday night, officials at the Three Gorges Dam have been pumping increased amounts of water through the sluices in anticipation of the coming flood.
Water flows in the upper reaches of the Yangtze river rose after the heavy rain at the weekend.
The surge was predicted to reach the dam on Wednesday morning.
The authorities say it is likely to be slightly smaller than that seen at the dam last week though.
Washed away
In one village in Henan province in central China, 33 bodies have been recovered after a bridge collapsed.
The local government says the water in the river under the bridge was usually less than a metre high.
But the area was lashed by the heaviest rain in a hundred years.
Water levels rose substantially and the river was flowing 63 times faster than normal when the bridge was washed away completely.
The accident happened on Saturday but has only just been reported by the national media.
China says disasters caused by the bad weather have left more than 1,250 people dead or missing.
The economic losses are estimated to be more than $22bn (£14bn).
