Workers set explosives on a dam formed by this month's catastrophic earthquake in Sichuan province, after thousands of soldiers finished an enormous drainage channel after 10 days of frantic digging.
Water trapped in the lake in Tangjiashan was expected to start flowing out on Saturday afternoon, said Luo Haolong, a commander of the water resources section of the armed police working on the site.
"This afternoon, the water will flow again," said Luo. A top party official earlier told AFP that the water would start flowing in coming days.
More than 197,000 people have been evacuated in case of flooding, an official from nearby Mianyang city told AFP, and soldiers who had been working round-the-clock were also leaving the area.
"Right now, I believe the lake problem is under control, we do not have any fears that there will an uncontrollable flood," he said.
But there was still concern for the more than a million people living downstream, with enough water to fill 50,000 Olympic-sized swimming pools contained in the lake.
Local residents said people had been ordered to move at least 30 metres above the level of the river, as a precautionary measure. The Mianyang official said a total of 1.3 million people how to evacuate if the order was given.
"They are not going to evacuate if the water flows peacefully as planned," she said.
Dealing with the "quake lake" has become one of the most pressing issues in the aftermath of the May 12 earthquake that killed more than 68,500 people, with over 18,000 still missing.
Threats to the environment if the dam burst remained a concern, with state media reporting that radioactive and chemical materials were being moved from the path of the quake lake if it burst.
However, it was not the only area of southwest Sichuan at risk.
There were 33 other lakes created by the quake, 28 of which were at risk of bursting, the official Xinhua news agency reported earlier.
Companies dealing with environmentally sensitive materials like chemicals across Sichuan were also a cause for concern.
The Ministry of Environmental Protection said late Friday that authorities had dispatched thousands of people to inspect businesses in quake-hit areas, finding some with possible environmental risks.
Of 14,357 companies, including some 2,900 chemical firms, surveyed in Sichuan province, inspectors found 134 potential risks, Xinhua news agency said, quoting a statement on the ministry's website on Friday.
Nearly 30 of the potential risks had been dealt with.
The ministry also said the province's environmental quality remained stable and water was acceptable for drinking.
Damaged buildings that were still standing also remained a risk – schools in particular – after thousands of children were killed when their school buildings collapsed in the tremor.
Local authorities were ordered to conduct safety appraisals of all remaining school buildings in the zone, to make sure students going back to their studies were safe, Xinhua news agency quoted the State Council as saying.
Those buildings whose construction design violated the law would be dealt with according to the law, the State Council said.