Four dams created by earthquake rubble broke Tuesday in south-western China, sending masses of water downstream, reported dpa.
The water was expected later in the day to reach the Sichuan province city of Mianyang, where 250,000 people had already been evacuated, the official Xinhua news agency reported. An emergency plan calls for more than another 1 million people to evacuate.
The dams broke below the largest of the 30 so-called quake lakes, created when landslides in the May 12 magnitude-8 tremor blocked the flow of rivers.
Water levels at that unstable lake, at Tangjiashan, have been steadily rising, threatening more than a million people downstream.
Authorities have been seeking to drain the lake, and the flow to a drainage channel drastically speeded up Tuesday after engineers blasted boulders that had been slowing the outflow, Xinhua said.
The water level fell overnight by more than 5 metres, Xinhua reported.
The water is laden with earthquake debris, including household appliances, and bodies of earthquake victims.
Nearly 70,000 people have been confirmed dead in the May 12 quake, according to the government, and nearly 18,000 are missing.