June 11 (Bloomberg) -- China's military drained half the waters from a lake formed after the May 12 earthquake in Sichuan province, removing the threat of flooding of a city downstream and allowing the return of evacuees within days.

A ``decisive victory'' was achieved, Liu Qibao, the province's Communist Party chief, said late yesterday, state-run Xinhua News Agency reported.

A channel dug by the military to drain Tangjiashan lake widened as the flowing water washed debris downstream, Xinhua said, citing relief headquarters in the area. State television showed residents of Mianyang, a city of 5 million people about 50 kilometers (30 miles) downstream from the lake, watching a swollen river flowing within its embankment.

Tangjiashan lake is the biggest of more than 30 that were formed in the valleys of Sichuan when mountainsides collapsed during the quake. About 250,000 people ordered out of areas close to the river downstream will be able to return to their homes within days.

``The best situation is to completely clear out the water in the Tangjiashan lake before the flood season'' later this month, Water Resources Minister Chen Lei said yesterday. ``We'll take advantage of the time to eliminate the risk as soon as possible.''

The lake's volume fell to 110 million cubic meters (29 billion gallons) by 5 p.m. yesterday, compared with more than 240 million cubic meters the day before, Xinhua reported.

Preparations had been made to evacuate as many as 1.3 million people from parts of Mianyang and towns and villages nearer the lake.

Controlled Explosions

Two controlled explosions on June 9 removed the biggest blockages in the channel, which is 475 meters (519 yards) long, and allowed the water to start flowing fast enough to widen the channel by its own momentum.

Workers are digging another sluice in the barrier to increase the drainage, Xinhua said yesterday.

Authorities were concerned aftershocks or rain would trigger landslides and raise the lake's water level too quickly or cause the barrier to collapse.

A 4.8-magnitude aftershock struck at 6:23 a.m. local time today, the U.S. Geological Survey said on its Web site.

The death toll from the quake remained at 69,146 people, with 17,516 missing, both unchanged since yesterday, the army said in a press briefing today.

The 7.9-magnitude earthquake was the most powerful to hit China since a magnitude-8.6 quake struck Tibet in 1950, killing 1,526 people.

To contact the reporter on this story: Aaron Sheldrick in Tokyo at asheldrick@bloomberg.net.

Last Updated: June 11, 2008 04:18 EDT