Water began to be discharged on Monday from a lake that had formed behind a landslide on the border of Sichuan province and the Tibet autonomous region.
A man-made floodway was created to channel the water and prevent a secondary disaster if the natural dam were to burst, flood control authorities said.
The discharge reduces risks as the lake's water level continues to rise, said Xu Chong, a researcher at China Earthquake Administration's Institute of Geology. Backed-up water is threatening the upper reaches of the Jinsha River, and poses a potential danger downstream if it were to break out.
Since it formed on Oct 11, the lake has retained about 524 million cubic meters of water. It was created when two landslides－one in Chamdo, Tibet, and the other in Garze Tibetan autonomous prefecture in Sichuan－blocked a section of the Jinsha River.
Water levels had returned to normal on Oct 14 with no injuries reported. But the second landslide blocked the river again on Nov 3.
Water entered the 220-meter-long floodway at 4 am on Monday and began flowing downstream. It could be observed in the lower reaches about seven hours later, according to the Ministry of Emergency Management.
The upper and lower reaches of the Jinsha River had not seen any dramatic change in water level as of 3 pm, according to the Changjiang Water Resources Commission.
China's flood control authorities alerted areas along the river, including Tibet and the provinces of Sichuan and Yunnan, about potential risks and asked local authorities to ensure public safety and minimize potential damage.
Traffic has been banned in vulnerable areas until the emergency response concludes.
Some 34,200 people were evacuated along the river in Sichuan and Tibet as the lake's water level continued to rise, the ministry said on Sunday.
The barrier lake had inundated a farmhouse and a suspension bridge in Sichuan's Jinsha township, Xinhua News Agency reported on Saturday.
Fifty-two barns, as well as farms and roads, were damaged, while 25 homes, the township government building, a school and a clinic are threatened, the report said.
Xu said the landslide followed long-term geological deterioration.
"Landslides that result in barrier lakes can cause much greater damage than the landslides alone," he said.
The researcher noted that global warming－though some scientists disagree－could lead to more landslides that block rivers in the area, as happened on the Yarlung Zangbo River in Tibet last month.