HANWANG TOWN, China — Tens of thousands of people poured down from the mountainsides of Sichuan yesterday searching for food and water as rescuers evacuated towns where over 40,000 people are dead, buried, or missing.

Helicopters began to airlift residents out of the villages of Wenchuan county at the epicenter of the magnitude 7.9 earthquake, which struck China on Monday, while others walked for hours into the valleys and plains to the south and east. The county is still cut off from land-based relief by landslides.

But the exodus could yet be heading into more danger, with emergency workers attempting to plug gaps in two major dams.

The Zipingpu dam, above the city of Dujiangyan with its population of 600,000, was said at one stage to be in an “extremely dangerous state” with cracks more than 4-inches wide appearing in its face, though the water resources ministry later said on its Web site that it was stable for the time being. Tulong dam, further north on the Min river, was said by officials to be near collapse, something that would have a knock-on effect on a series of dams and power stations further downstream.

Almost 400 dams were said to have been affected by the quake — the wet and mountainous province is crisscrossed by some of China’s biggest and fastest-flowing rivers.

While some dams, such as Zipingpu, are modern, others were built when construction standards were lower.

All day long, the 100,000 troops now assigned to the rescue effort grappled with the wreckage in major cities, towns and villages across a wide area.

There were some successes, including the rescue of a 3-year-old girl in Beichuan, one of the worst-affected towns, and an eight-months pregnant woman trapped under an apartment block in Dujiangyan.

But by the evening the death toll stood at nearly 15,000, with another 25,000 reported buried and more unaccounted for.