China has warned that one of its longest waterways, the Yellow River, is in danger of bursting its banks.
The warning follows heavy rain in the north of China.
The south has also been hit by bad weather. Dozens of people are dead after heavy storms, and more than a million have had to leave their homes.
At least 20 people have died in the province of Guangdong, and the authorities there say economic losses are extensive.
More rain is expected to batter both regions in the coming days.
The national meteorological service warned that the 5,500 km (3,400 mile) Yellow River might see "quite large" floods this year, according to the state news agency Xinhua.
Millions of people live along the river, but according to the BBC's Dan Griffiths in Beijing it is not clear how many might be affected if the river breaches its banks.
The river is the second longest in the nation, and is often referred to as China's sorrow because of its long history of devastating floods.
Elsewhere torrential rain has continued to batter several provinces in southern China, leading to widespread flooding.
At least 57 people have died, and 1.27 million others have fled their homes, state media reported on Monday.
The flooding has submerged large areas of farmland and destroyed thousands of homes in Guangdong and Jiangxi provinces.
The flooding in the Pearl river delta is the worst for 50 years, submerging large areas of farmland and destroying thousands of homes in Guangdong, and prompting the local government to issue an emergency flood alert.
"A major flood is feared if rain continues," Huang Boqing, deputy director of the province's flood control and drought relief headquarters, told the China Daily newspaper.
In all the torrential downpours have affected nine provinces - one of which, Sichuan, is still reeling from last month's massive earthquake.
Some 87,000 people were killed or reported missing after the 12 May earthquake.
China 's rainy season causes chaos every year, often leaving many dead and forcing millions to leave their homes.
In the past, the Chinese authorities have warned that climate change may make the problem worse, our correspondent says.