At least 127 people have died and an estimated 2,000 are missing after heavy rains triggered landslides in mountainous northwest China's Gansu province.
The silt and mud clogged the Bailong River, which overflowed its banks.
Approximately 40,000 residents have been evacuated, China Central Television reported, but the landslides are hampering rescue efforts.
According to China's state news agency, Xinhua, many houses in Zhouqu county have collapsed, and streets and roadways are submerged under three feet of water.
"Torrential rains began around 10 p.m. On Saturday. Then there were landslides," Diemujiangteng, head of the county, told Xinhua.
"Now the sludge has become the biggest problem to rescue operations. It's too thick to walk or drive through," he said.
The government called up 3,000 soldiers to help in relief efforts, China's Ministry of National Defense said in a statement on Sunday.
Chinese Premier Wen Jiabao traveled to the north from Beijing on Sunday, according to Xinhua
In the southern part of Gansu province, power outages made it difficult to assess how many of the missing were in imminent danger from the widespread flooding and landslides.
Gannan district head Mao Shengwu said in some places the water levels were rising by a meter (approximately a yard) every five minutes.
There are families standing on the roofs of their submerged houses waiting to be rescued, he said.
More than 1,450 people have died during floods this year, and an estimated 670 are still missing.
AP contributed to this report.