BEIJING, China (CNN) -- Authorities expected water to begin draining Thursday from a lake created by last month's devastating earthquake in southwestern China.
But continuing aftershocks remain a concern: they could destabilize the rock and soil that form a quake-created dam holding back more than 200 million cubic meters (50 billion gallons) of water.
Four earthquakes measuring between magnitude 4.5 and 5.3 rattled the region during a 24-hour period, ending Thursday afternoon.
Engineers were anxiously watching as water rising behind the dam neared a spillway designed to relieve the pressure on it.
As of Wednesday afternoon, water in the so-called Tangjiashan quake lake had risen to within 1.4 meters (4.5 feet) of the spillway, in Beichuan county, Chen Lei, minister of water resources, told the Xinhua news agency.
"We must prepare for dealing with the worst situation but strive for the best results," said Chen.
The lake holds enough water to fill roughly 81,500 Olympic-size swimming pools.
The aftershocks from the May 12 quake and the threat of more rain, meanwhile, put the area in "extreme danger," Liu Ning, chief engineer of the Ministry of Water Resources, told reporters Wednesday.
Heavy rainfall and severe aftershocks could compromise the integrity of the dam and send water cascading out. In the event of particularly heavy rain and a major aftershock, the chances of a catastrophic failure would stand at 93 percent, Liu estimated. Watch report from quake lake command center »
Authorities have evacuated about 200,000 people who are considered to be at risk if water overflows the dam. Engineers built the spillway to drain water from the lake in a controlled way once it tops the lowest levels of debris blocking the Jianjiang River.
Engineers used heavy equipment and dynamite to chisel a 475-meter-long (1,560 foot-long) spillway through tons of rock and soil that slid off the hillside during the earthquake. They finished work on the channel Saturday.
Thirty-five so-called quake lakes were created by the 7.9-magnitude quake in Sichuan province
The Tangjiashan lake worries Chinese officials most because it could threaten up to 1.3 million people downstream.
The official death toll from the quake has risen to 69,127, China's Civil Affairs Ministry reported on Thursday, an increase of 5.
Another 373,606 people were injured and 17,991 remain missing, according to the ministry.