China's enormous Three Gorges dam remains on high alert as waters from the flooded Yangtze river continue to pour into the reservoir behind it.
Life in Chongqing has been badly disrupted
Water levels were expected to peak on Wednesday, but officials insist the massive dam is able to cope.
Floods and landslides caused by heavy rains upstream from the Three Gorges have already killed 160 people.
Navigation through the dam's locks has been suspended for the first time since operations began earlier this year.
The Chinese authorities are reinforcing the structure's outlying bulwarks to deal with the fast-rising waters.
The massive dam project, built at a cost of $25bn, is supposed to help control China's annual flooding.
But it continues to be highly controversial, even though China desperately needs the power it is designed to generate, says the BBC's Francis Markus in Shanghai.
THREE GORGES DAM
World's largest water control project
Designed to produce hydroelectric power and curb flooding
Dam has 26 generators, which will eventually produce 18.2m kilowatts of energy
When fully completed, it will create a reservoir 600 km long
More than 600,000 people forced to relocate from the area
According to Ute Collier, the dams initiative leader for the World Wildlife Fund, the main danger concerns people living below the dam.
As water fills the reservoir from the swollen rivers above, the level has to be reduced by releasing extra water downstream to alleviate pressure on the walls of the dam.
"This could become a serious problem if the rains continue," Ms Collier told BBC News Online.
Villages in Hubei province, downstream on the Yangtze
River, are said to be braced for a possible surge in water levels.
Ms Collier says part of the reason China suffers such acute flooding on the Yangtze is because huge areas of land have been deforested, leading to run-off water filling the rivers, which are then unable to cope when there is heavy rainfall.
"There needs to be an effort to allow natural flood plains to do their work," she said. "Artificial dams have trouble dealing with extreme events like this."
Once a century
Upstream from the Three Gorges, floods and landslides unleashed by torrential rain have already left 160 people dead.
Many others are missing and thousands are reported to be injured or sick, following five days of incessant rain that has swamped rivers in a wide area of south-west China.
In some areas of Sichuan province and Chongqing municipality, the flood waters have been so deep that they have submerged four-storey buildings. Entire communities have been marooned.
More than 450,000 people have been evacuated and 127,000 homes destroyed or damaged, the China Daily newspaper reported.
The worst-affected area is said to be the city of Dazhou, which was hit with 360mm (14.4 inches) of rain.
Thousands of military personnel, police and other rescue workers are helping to distribute food and medicine in the area.
The official Xinhua news agency described the situation as a "catastrophe which is not likely to happen in a century".
Meteorologists warn that at more
major storms will strike the region in the coming weeks.