The threat of flood forced an urgent evacuation of 20,000 people living downstream of three earthquake-formed lakes detected on Thursday by helicopter reconnaissance in quake-hit Dujiangyan City in southwest China's Sichuan Province.

The hydrological and meteorological departments in Sichuan issued a flood warning on Friday, forecasting that the summer flood is likely to be the biggest in a decade and come at the beginning of July, which is earlier than in past years because of the effect of abnormal rainfall in May.

Precipitation in Sichuan between May and June was 30 percent to 70 percent more than that of the same time last year, said China Meteorological Station earlier this month.

The flood threatens over 10 million Chinese in the earthquake areas, many living in tents and makeshift houses, some 40 days after the magnitude-8 quake hit the province, killing some 70,000 people.

The earthquake is a challenge to flood control in Sichuan this year, as quake-damaged reservoirs and river levees are now more vulnerable, said the provincial headquarters of flood control and drought relief.

"If the flood comes, newly-built temporary settlements for homeless quake survivors in low-lying plain will be inundated," said worried officials in Aba Tibet and Qiang Autonomous Prefecture, where the earthquake's epicenter of Wenchuan County was.

They moved over 110,000 quake survivors out of the mountainous areas to a 20-km valley strip to protect them from secondary geological disasters such as landslides and mud flows in the wake of the earthquake.

If embankments could not hold flood water in Minjiang River, they would have to carry out another round of massive evacuation of people living in tents and makeshift houses at any moment.

"We have cancelled the building of temporary settlements for earthquake victims in 12 villages in Yingxiu town, because of the flood threat," said Xiang Enming, a disaster-relief director from south China's Guangdong Province, who is helping reconstruction in the quake-hit township in Wenchuan.

Xiang, speaking on Thursday, said they would have to find a safer place to evacuate the 5,800 villagers, but a new site has not yet been chosen.

Latest figures from the flood control headquarters suggested that the earthquake has formed 34 lakes and damaged 1,803 reservoirs and 495 sections of damaged embankments in Sichuan.

"Frequent landslides along mountain slopes can produce new quake lakes at any moment. There are 56 blockage of rivers in Mianyang City," said An Tianyun, director of the municipal water affairs bureau.

The city is home to the largest of the quake lakes in Tangjiashan, which had forced an evacuation of more than 250,000 residents in Mianyang alone, before drainage succeeded in reducing its risk earlier this month.

However, An said that as the quake has shaken loose the geological structure, and heavy rainfall would exacerbate the disasters.

The earthquake and its aftershocks have caused severe damage in 18 counties in Sichuan, with three of them in Aba prefecture being the worst hit. The Plateau Meteorological Research Institute in Chengdu warned the pending summer flood, in tandem with heavy rains, would cause more massive and devastating geological disasters in the quake-hit region.

In Mianzhu and Deyang cities, rehearsals of urgent evacuations have been staged, while water affairs departments closely watch changes in rivers and lakes.

The province has mobilized round-the-clock patrolling on dangerous sites for geological disasters and flooding, with the intention of providing early warnings to minimize casualties.

(Xinhua News Agency June 28, 2008)