Chinese troops have begun slowly draining a swollen lake at Tangjiashan, formed behind a landslide after the 12 May earthquake in Sichuan province.
Experts warned the lake could burst at any time and water is now being drained through a man-made sluice and channel.
Engineers have been monitoring bridges and river banks downstream to see if they will hold.
More than 250,000 people have already been evacuated to higher ground.
Chinese officials say refugees will have to wait until the threat of flooding has subsided before they can go home.