BEIJING, China (CNN) -- Fears were mounting Monday for the safety of one million people downstream from a "quake lake" in China as waters threatening to burst a dam of landslide rubble were rising faster than engineers could drain them.
Authorities are concerned the Jianjiang River will top the dam unless efforts to dig and blast out emergency draining channels can drastically decrease the water level.
But engineers also risk letting out too much water at one time, which could also trigger a dam collapse.
"Increasing the outflow of water is critical for the dam's safety," said Zhang Ting, head of the Sichuan provincial hydro-meteorological bureau, the Xinhua news agency reported.
"If the water flows too slowly, the inflow will increase the pressure on the dam. But again, too voluminous an outflow can erode the diversion channel and cause the dam to collapse."
Water is flowing into the lake more than four times faster than it's flowing out, Zhang said.
A 475-meter-long (1,560 foot-long) spillway for the water was completed more than a week ago, but engineers enlarged it over the weekend to increase the flow.
Chinese security forces have since dug a second channel and are working on a third, said Liu Yongjian, an army officer in charge of the work, the Xinhua news agency reported.
The so-called Tangjiashan quake lake formed after last month's devastating magnitude 7.9 quake. Tons of rock and soil slid off a hillside, blocking the river.
Aftershocks continue to pepper the devastated region.
A pair of magnitude 5.0 quakes have rattled eastern Sichuan province over the last 24 hours.
Officials are concerned the building water pressure combined with the aftershocks could destablize the dam and send billions of gallons of water cascading down the river valley. Heavy rainfall could also strain the dam.
It could threaten up to 1.3 million people downstream. More than 250,000 people already have been been evacuated, Xinhua reported.
More than 69,000 people died in the May 12 quake, but more than 17,000 remain missing, according to the official government toll.