Drainage of the dangerous Tangjiashan "quake lake" in Sichuan Province has gone smoothly since Saturday, but the local government remained on on high alert, state-run media said Sunday.
Army engineers fired missiles to blast boulders in a man-made sluice channel to accelerate water drainage, Xinhua news agency said.
The agency said water flowing through the sluice channel widened from previously less than five metres to about 9 metres, the dpa reported.
"Generally speaking, construction of the lake's drainage projects goes on well, but the lake remains dangerous for hundreds of thousands of people downstream," said General Ge Zhenfeng, deputy chief of staff of the People's Liberation Army, who was supervising work at the site.
"It will take us a few days to eliminate the potential danger of the lake," the general said.
About 600 police and soldiers worked for six days and nights to dig a 475-meter channel to divert water from the lake.
Xinhua said soldiers were still widening and deepening the sluice channel with the help of some 30 bulldozers and excavators. They were also digging a second sluice channel on another side of the lake barrier.
The soldiers have finished building one third of the new channel, which needs to remove about 60,000 cubic meters of earth and stone, said Liu Yongjian, a PLA officer in charge of the channel projects.
"We have also prepared for an underwater explosion to deepen the channels for accelerated drainage," said Liu.
The lake started to drain on Saturday morning and water flowed at a speed of more than 10 cubic meters per second, far more than the previous two cubic meters per second, according to the quake lake relief headquarters in Mianyang City, the agency said.
Landslides could occur on mountains not far from the lake, which might pour another 17 million cubic meters of rocks and earth into the lake. If it happens, the lake's barrier would immediately burst and workers on the barrier could be swept away, the report said.
The "quake lake" was formed after a massive quake-triggered landslide from Tangjiashan Mountain and blocked the Tongkou River, which runs through Beichuan County, one of the areas worst hit by the quake.
Holding more than 220 million cubic meters of water, the swollen lake is the largest of more than 30 quake lakes in Sichuan following the May 12 quake, posing a threat to 1.3 million people downstream, Xinhua said.
More than 250,000 people in low-lying areas in Mianyang have been relocated under a plan based on the assumption that a third of the lake volume breached its banks.