Dramatic pictures as billions of gallons of water are released from China's quake lakes

Water flowing quickly out of Tangjiashan lake, flooding low-lying areas of the devastated town of Beichuan just downstream.

Residents in the area were previously evacuated and others along the river were preparing to leave if waters rose too high.

Forty per cent of homes in the town of Qingyi were at risk while the city of Mianyang, where residents have been practicing evacuation drills, was on alert.

Befoer and after: A general view of the earthquake-hit city of Beichuan before the flooding and, today (below), after a 'controlled' drainage operation

Military alert: Soldiers prepare to destroy floating objects obstructing the water drained from the Tangjiashan "quake lake" using anti-tank weapons

Flood waters seeped into riverside houses in the largely evacuated town of Qinglian, a resident said.

'Everybody feels lucky that it didn't submerge the streets and the neighborhood," said Wu Zhenxing.

For days, troops have been using dynamite and anti-tank weapons to blast boulders and other obstacles in a man-made diversion canal, trying to speed the flow of water and relieve pressure on the lake's unstable mud and rock banks.

It wasn't immediately clear whether engineers could control the flow and prevent a breach that would cause severe flooding downstream.

More than 250,000 people downstream have already moved to high ground due to concerns that the barrier holding back the lake could break apart.

Threat: Witnesses said water levels in the river running through Beichuan rose dramatically this morning

Draining away: Muddy, brown water from a quake lake in southwest China was pouring into the flattened town

A total of about 1.3 million people live in the downstream area.

David Petley, a geography professor at the University of Durham who has been monitoring the lake, said it appeared the flow of water was out of control.

'The government absolutely must evacuate everyone at risk downstream as there is a really high chance of a total collapse. I want to emphasize that it seems to me that this is a crisis situation,' Petley added.

Petley earlier said the top of the bank was holding, instead of eroding slowly as it should, while the channel further down was eroding too quickly.

Trees, barrels, TV sets, refrigerators 'and the occasional dead body' could be seen in waters pouring out of the mountains.

Rising tide: Chinese troops inspect a quake lake prior to firing missiles and using dynamite to clear more diversion channels