Chinese troops in Beichuan county, where the authorities are fighting to relieve pressure on a quake-created lake.

Tangjiashan Lake did not exist before the 7.9-magnitude earthquake that struck mountainous Sichuan province May 12. The quake caused a landslide that dammed Jianjiang River and created the lake.

Now crews are working 24 hours a day to clear debris and dig a channel so water from the lake can flow in a controlled way to the lower reaches of the river.

Workers hope the spillway will allow the water to bypass Beichuan town. Farther downriver, in the town of Mianyang, the government was preparing to evacuate the population of 1.3 million people if the lake bursts its banks.

Some 200,000 people have already been ordered to leave at-risk areas downstream.

The lake in Beichuan County is the largest of several "quake lakes" created by the earthquake nearly three weeks ago.

The lake's water level has continued to rise about 2 to 3 meters every day. Authorities are concerned that the lake could burst its banks or that another aftershock or earthquake could rupture the earthen dam and cause a sudden flood of water on the communities downstream.

Some of the mountains overlooking the region's rivers are now bare and brown where landslides wiped them clean of their tree-covered sides. Rocks can still be heard falling from the mountains.

It will be Wednesday or Thursday before the Chinese authorities know whether their emergency operation will be successful, a commander said.

Because the region is so remote -- most of the roads were cut off by landslides -- the government has brought the equipment and personnel in with Russian military helicopters.

Another challenge is the weather. The commander said some members of the Chinese army have resorted to walking to the site, carrying fuel on their backs, because the helicopters were unable to fly in the fog and rain.

Many of those who have already left have been living in tents after losing their homes in the earthquake, forcing them to pick up and move again -- some for the third time.

After clearing out of one area, people left their tents and tarps standing, saying they hope they will be able to return there.

The government told frustrated evacuees that they were creating a safer place for them to stay. However, the residents said the government-created site is not as desirable as the site they are being ordered to leave. They say they would like to return once floodwaters recede.

The official death toll from the quake stands at 68,858 as of Friday, with another 18,618 people missing.