Torrents of muddy brown water were today gushing out of a dangerously unstable lake that was created when landslides set loose by China’s deadly earthquake this month blocked a river upstream from the devastated town of Beichuan.

Desperate to find a way to release the rising waters before the force of the swelling "quake lake" broke the dam and crashed down into the plains below, China’s army began blasting the earth with explosives and anti-tank missiles in the last few days.

The explosions widened a sluice dug over the past 10 days by dozens of People’s Liberation Army soldiers who had been dropped by helicopter on the inaccessible dam.

The swirling waters swept clumps of trees and abandoned vehicles towards the abandoned town of Beichuan where 8,600 of the 13,000 residents were killed by the May 12 tremor, the deadliest to shake China in a generation.

The water level in the Tangjiashan quake lake dropped by 13 metres (43 ft) in the course of this morning. The lake, the largest of the more than 30 created when landslides triggered by the quake blocked the flow of rivers, has so far prompted the evacuation of more than 250,000 residents downstream in case the mud-and-rock dam bursts.

Survivors made their way to the tops of surrounding hills to watch as their town, already flattened by the earthquake, was swamped by the turbulent waters.

The drowning of swaths of what was once the main town of Beichuan county brought fresh grief for those who had held out hope of finally recovering the remains of missing family and friends or of finding some of their valuables and possessions buried in the rubble.

Wang Guiru, 43, had hoped to find the remains of his wife, who died alongside his father and mother-in-law. He said: "Now I guess we can never go back. This is fate. We have to learn to face up to realities."

Further downstream near the city of Mianyang, the river had turned into a torrent several hundred metres across, bursting banks in places. Fu Anyun stood watching. "I never thought it would rise so quickly. We've been waiting and waiting, but when it finally came it was still a shock. Now all we can do is watch and see if our homes our flooded or not."

Helicopters were evacuating remaining soldiers and experts from the top of the dam, which was no longer safe with cracks appearing as the flow of water accelerated by more than tenfold. State media attributed the abrupt increase in water discharged from the lake to two massive blasts late on Monday that broke through the bottleneck in the sluice opened by the soldiers.

Zhou Hua, a spokesman of the lake relief operation, said: "The flow downstream has increased dramatically, but the dam hasn't collapsed. The channel has widened but this isn't a collapse. So far everything is happening within expectations. As things are, we don't expect to have to evacuate any more."

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