China has ordered nationwide safety checks for schools a month after the Sichuan earthquake saw many collapse in heaps of rubble.

Muddy lake water from a dangerously unstable "quake lake" rushed into the devastated Chinese town of Beichuan on Tuesday, covering about a third of the settlement where the water level was rising fast.

Chinese troops are carving a third drainage channel into the unstable dam holding back a big "quake lake", as water levels rise and aftershocks send more debris tumbling into the water, state media reported on Monday.

Chinese troops began easing pressure on a dangerously swollen "quake lake" on Saturday, with water gushing into a man-made sluice in an operation monitored by satellite.

China readied on Friday to ease pressure on a swollen "quake lake" threatening hundreds of thousands of people downstream in the southwestern province of Sichuan.