(Adds Huaneng warning in paragraphs 4-5)

By Chris Buckley

BEIJING, May 15 (Reuters) - The earthquake that devastated southwest China has left many dams badly damaged, posing an unknown threat of collapse and flooding made worse by poor management and information, the country's water minister said.

In Sichuan and Chongqing, at least 17 reservoirs have been damaged, with some dams cracked or leaking water since the quake struck on Monday, according to state media. Several are on the Min River, which tumbles through the worst-hit areas between the Tibetan plateau and the Sichuan plain.

Chinese state media said troops rushed to repair cracks in the Zipingpu Dam in Sichuan province caused by the earthquake on Monday. The dam is now deemed structurally stable.

An official from Huaneng Power Group told state television on Thursday that contact had finally been made with 110 people trapped at the Taipingyi hydropower plant near the badly damaged city of Yingxiu, but would have to evacuate them by air.

"The big problem is the dam faces a danger of collapsing and the water level has kept rising," Zhang Ping told CCTV.

Water Resources Minister Chen Lei said such damage was widespread and sounded far from assured in comments put on the ministry Web site (www.mwr.gov.cn) on Thursday.

"Especially in Sichuan province, there are many dams, damage from the quake is extensive and the hazards are unclear," Chen said in a speech to officials a day earlier.

"Blockage of waterways can be extremely sudden, and draining them is difficult."

And the minister blamed more than nature for the dangers.

"Because the management systems of hydro-power stations is not smooth and information channels are blocked, the extent of their damage is unclear," Chen said.

RIVERS BLOCKED

The mountainous western half of Sichuan has been a magnet for hydro-power projects, hoping to profit from China's huge thirst for energy.

As China struggles to cope with what Xinhua news agency said could be a death toll of more than 50,000, the vulnerable dams are an additional threat. One burst could unleash a cascade that threatens other dams and residential areas.

A second threat is water backing up behind landslides. At least two rivers -- the Jialing in southeast Gansu, which flows into Chongqing, and the Jian river near Beichuan, one of the worst-hit cities -- are blocked in this way.

"The river looked completely dry, and there were landslides into the riverbed beyond the town," said John Garnaut, a reporter for the Sydney Morning Herald visiting Beichuan.

Earlier on Thursday, fears that the river could burst and inundate the flattened town forced rescuers to briefly suspend work and evacuate to higher ground, media reported.

Chen ordered officials to put in place plans to evacuate endangered people if dams threatened to burst.

Water has been released from the damaged Zipingpu reservoir, to avoid "disaster" to the city of Dujiangyan downstream if it burst, the water ministry said on Wednesday. (Additional reporting by Lucy Hornby; editing by Nick Macfie and Andrew Roche)

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