By Lucy Hornby
BEIJING, May 14 (Reuters) - A devastating earthquake that struck China this week has raised concerns over the stability of dams and infrastructure in the afflicted mountainous area, while landslides have blocked a river that flows to the region.
More than 13,000 people have been confirmed dead and at least the same number remain missing or buried by the 7.9-magnitude earthquake, which struck to the northwest of Sichuan's provincial capital Chengdu on Monday.
Heavy rains could compound the damage by hindering rescue work, triggering mudslides and adding to pressure on weakened dams, officials and state media said on Wednesday.
The quake left a swathe of collapsed buildings in the steep, rainy mountain range that runs from southwest to the northeast. The dramatic shelf where the Tibetan plateau collides with the Sichuan basin is ideal for hydropower, while doubling the challenges of building roads, bridges and railways.
One of the worst-damaged cities is Dujiangyan, site of multiple dams and weirs that irrigate about 3 million hectares in the fertile Sichuan plain. The Dujiangyan irrigation works date from the 3rd century BC, when engineers split the Min river where it spills from the mountains, and diverted it to irrigation channels along the plain.
'Upstream on the Min river is an important reservoir called Tulong which is already imperilled. If the danger intensifies, this could affect some power stations downstream,' He Biao, deputy party chief of Aba prefecture, told reporters.
'This is an extremely dangerous situation.' The quake caused the 760-megawatt hydropower generating unit at Zipingpu, 9 kilometres upstream of Dujiangyan, to collapse, the provincial government said. It began operations in 2006, as part of China's program to develop its poorer western regions.
Water has been released at 50 percent more than average levels, to lower water levels in the Zipingpu reservoir and relieve pressure on the cracked dam, the Ministry of Water Resources said on its Web site (www.mwr.gov.cn).
'If Zipingpu develops a serious safety problem, it could bring disaster to Dujiangyan city downstream,' where half a million people live, the ministry said.
Cracks on the famous Yuzui or 'Fish mouth' levee further downstream, the crux of the Dujiangyan system, are not serious, the ministry said.
The ministry has dispatched teams to Sichuan, Chongqing, Yunnan, Gansu and Shaanxi regions to prevent dams that were damaged by the earthquake from bursting and endangering the lives of residents, it said.
'Local governments should monitor (dam) projects, to discover and repair damage as soon as possible. In case of danger, make sure to transport people to safer places,' Water Minister Chen Lei said, according to Xinhua.
The massive Three Gorges Dam, hundreds of kilometres (miles) downriver from the epicenter, was not affected by the quake, officials said.
The flow of the Jialing River has been effectively blocked in Huixian county, in southeastern Gansu's Longnan region, by landslides triggered by the quake, the Xinhua news agency said.
Rubble created a 6-meter tall, 30-meter wide and 100-meter long dam, holding back 600,000 cubic meters of water.
Huixian was the site of a collapsed tunnel, which caused a train carrying gasoline to catch fire along the Chengdu-Baoji line. Rails have buckled along the line, which curves through 200 tunnels to link Sichuan with Shaanxi province to the northeast.
(Additional reporting by Niu Shuping and Ian Ransom; Editing by Ken Wills and Sanjeev Miglani)