Officials have warned for years that heavy tree-felling and rapid hydro development were making a mountain area struck by China's deadliest mudslide in decades vulnerable to flooding and land slips.

Rescuers work in the landslide-hit Zhouqu County, Gannan Tibetan Autonomous Prefecture in northwest China's Gansu Province, Aug. 9, 2010. The death toll from rain-triggered mudslides in Zhouqu County has risen to 702 as of Tuesday afternoon, with 1,042 others still missing. [Xinhua]

Government reports last year urged work to restore a battered line of environmental defenses in a brittle landscape, deemed a "high-occurence disaster zone for landslides."

The Bailong River that rushes through the valley is swollen, some pent up behind a loose dam of landslide debris.

Officially, the landslide has been described as a natural disaster brought on by heavy rains, a drought that preceded it, and the huge 2008 earthquake that loosened the riverside slopes in Gannan prefecture, the mountainous region of southern Gansu.

But warnings about the fragile state of the river go back years.

Government documents issued before the disaster said the river and its surrounding slopes were prey to manmade problems, as the poor area sought to turn its forests and riverways into economic assets.

Specifically mentioning landslide risks in a document from April 2009, a government office overseeing the river highlighted the urgent need to rejuvenate the natural environment.

"Water volumes have fallen drastically, soil erosion is accelerating, and the trend of frequent land and mud slides and other geological hazards has not been contained.

"The repair of the Bailong River basin must not brook delay," the report warned.

Widespread tree-felling along the river since the 1950s had caused "grave destruction to the natural environment, creating serious erosion, worsening geological hazards, frequent natural disasters, a fall in water absorption capacity and shrinking flows," it said.

The Bailong River was over-exploited by rapid hydropower development on the Bailong River, said Zhang Qirong, an official with the Bailong River forestry authority.

The hydro dams appeared to have been a major factor - after loosening caused by the 2008 earthquake - in unleashing the landslides and clogging the river, he said.

In a recognition of the problems facing the river, the State Council in 2008 approved a plan to repair erosion and geological hazards there.