Zipingku's reservoir has a water capacity of 1.1bn m3
Earthquake survivors in Sichuan province face a serious threat of flooding because of earthquake damage to dams, officials say.
There are cracks on the surface of the Zipingku dam on the Min river, near the epicentre of this week's quake.
The hard-hit city of Dujiangyan, 10km (six miles) away, would be at risk if major problems emerge.
Water Resources Minister Chen Lei said there was further evidence of damage to almost 400 dams in the region.
He said there were also "prominent problems in safety and flood prevention" in reservoirs and hydropower stations in the affected areas.
The extent of the danger at hydropower stations remains unclear because management systems are "not smooth", he added.
Urgent attention is being paid in particular to medium-sized dams close to the town of Wenchuan, after an official warned that problems at the nearby Tulong reservoir.
The Zipingku dam is upstream from the Dujiangyan irrigation system, which has supplied water to Sichuan's fertile eastern plains for more than 2,000 years.
While the 156m-high (511ft) dam has been declared structurally safe, about 2,000 troops have been sent there to help with emergency repairs.
This could involve pouring soil into the river upstream that would then be sucked into the cracks and potentially seal them, Tom Foulkes, director-general of the UK-based Institution of Civil Engineers, told the BBC News website.
Chinese soldiers tackle 'extremely dangerous' cracks in Zipingku Dam
The authorities have also discharged large amounts of water from the reservoir, to help ease pressure on the dam wall.
However, experts have warned that the move was likely to swamp irrigation systems further downstream.
Aviva Imhof, a US-based expert with the organisation International Rivers, told the BBC that China had a poor record concerning collapsed dams.
A report on the group's website said the government had been warned of earthquake risks to the project before it was built.
Ms Imhof said that there were fears that aftershocks could lead to a devastating breach, leading to "untold destruction" downstream.
Mr Foulkes agreed that aftershocks were a real concern, but said that detailed seismic risk surveys were generally undertaken before beginning large dam projects in China.
"These fault lines are pretty well understood, and dams are generally designed with enough strength to cope," he added.
The dam was finished in 2006, and Mr Foulkes said it was of a type - made of rock with a waterproof concrete membrane - highly suitable for earthquake zones.
He said he was particularly concerned about older, smaller dams, often simply built from mounds of earth, which could be just as deadly if they collapsed.
Sichuan's two other major water projects, the Three Gorges dam and the South-to-North water diversion project, are said to be undamaged.
The earthquake zone is also home to China's chief nuclear weapons research laboratory at Mianyang and other testing sites.
China's Nuclear Engineering and Construction Corporation said on its website that several facilities in Sichuan had been damaged, but there was no mention of radioactive leaks.
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