Army units were working around the clock alongside police officials and civilians to shore up dykes and embankments on Dongting Lake, in the central province of Hunan, officials and state press said.
Others could be seen waiting in tents erected on top of embankments alongside mountainous piles of sandbags, ready to plug leaks as Dongting continued to swell despite clear skies.
Some lakeside houses were already totally submerged in parts of Yueyang, a large town next to where the Yangze River exits the lake, while locals negotiated flooded alleys in boats.
Dongting acts as a flood catchment for the historically flood-prone Yangtze, sparking fears for the safety of 7.5 million people in Wuhan, which sits on the river further downstream.
“If Dongting lake bursts its banks, then water flowing down the Yangtze could threaten neighbouring Hubei Province and its capital Wuhan,” the state-run China Daily warned.
Changsha, Hunan’s capital city of 5.5 million people on the Xiang River upstream from Dongting, “is in serious danger of being hit by floods”, the newspaper added.
As rain-swollen rivers emptied into the lake, around 850,000 were working on anti-flood measures throughout the region, it said.
Some people were already being evacuated, locals reported, following a province-wide declaration of emergency around Hunan province the previous day, which saw the army and police mobilised.
The International Red Cross on Thursday issued an urgent alert to donors to be prepared to send tents, quilts and other goods if the floods hit.
“Presently we have sent a warning to potential donors that (an appeal for donations) could happen ... We sent out a warning to donors to be prepared,” said Niels Juel of Red Cross in Beijing.
Dongting was like “a balloon filled with water” which could burst at any moment, he said.
“Potentially it could become very serious.”
Officials said Dongting could keep rising for three more days.
“If skies stay clear, the peak is expected to happen around 8am (6am PST) on Sunday,” said a provincial government spokesman.
At Chenglingji, a historical danger spot where the Yangtze exits the lake, waters were little more than a metre below levels recorded in 1998, when more than 4,100 people were killed in floods around the country.
Around 900 people have already died in floods and landslides this summer, including more than 100 in Hunan this month alone.
In the wake of the 1998 disaster, Beijing ordered a massive programme of anti-flood reinforcements around Dongting and the Yangtze, which are facing their toughest test yet.
But for some of the most vulnerable people, the precautions have already proved insufficient.
Hu Junxiang, her husband and three children abandoned their flooded home earlier this week, carrying what furniture they could to a cramped building further from the lake which they share with three other families.
“We moved here Thursday. If the waters rise even further we may have to move again,” she said mournfully.
In Yueyang, 35-year-old shopkeeper Zhou Xiong spent a third day trying to salvage goods from his ground-floor business and worrying about how to sustain a family of six.
“This is the worst flooding since 1998,” he said, looking out at more distant homes where lake waters were lapping at the roof eaves.
However Jian from the Hunan province government insisted that if the downpours held off, most people would be safe.
“If there is no rain in the next few days, there will be no danger,” he said.—AFP