"If the water level rises by another 2 to 3 m, it will overflow and threaten more than 2,000 people who are staying in shelters downstream," one of the experts said.
Water had already overflowed into the streets of a village neighboring Qianjin township, he said.
"If the lake bursts, it will form at least a 40-km waterway, and the results will be grave," he warned.
Experts were yesterday considering plans to explode the river embankment to divert the water.
A lake formed on the upper reaches of Jianjiang River in Pengzhou, however, breached yesterday afternoon. All residents downstream were evacuated earlier.
Water conservancy facilities in Sichuan have been severely damaged in the quake and the losses are estimated at 5.2 billion yuan ($740 million), according to the provincial water resources department.
As of yesterday, 88,615 water facilities had been destroyed, affecting 3.62 million people in the province.
Damaged power transmission lines in the rural areas added up to 1,978 km, affecting 1.16 million people.
Local authorities are repairing power supply and telecommunication facilities, and the Baoji-Chengdu Railway, paralyzed after a 40-car freight train derailed inside a tunnel in Gansu province.
In Sichuan and neighboring Chongqing, at least 17 reservoirs have been damaged, with some dams cracked or leaking water. Several are on the Minjiang River, which runs through the worst-hit areas.
The Lianhehua dam, built in the late 1950s northwest of Dujiangyan, had cracks big enough to put a fist in.
"When the dam is in this shape, we cannot relax," said farmer Feng Binggui who moved from his village below the dam into the hills.
Fan Xiao, an engineer with the Sichuan geology and mineral resources exploration organization, said he was very concerned about the devastating consequences of collapsed reservoirs and bursting lakes.
"The flood might cause greater damage than the quake as most of the hardest-hit areas are in the river valleys surrounded by high mountains."
Fan said hundreds of reservoirs were damaged in Sichuan and was especially worried about smaller ones, many of which are poorly maintained.