More than 15,000 people including troops and police were using sandbags and other materials to try to shore up dykes along the river.
Officials rushed to seal the first breach in the dyke that expanded to 400 metres, swallowing chunks of the 82-km-long barrier protecting a low-lying farming area with about 150,000 residents, Xinhua said earlier.
By late Tuesday, troops and officials had evacuated more than 90,000 people to safer areas, while other residents remained trapped by water and waited for rescue, the China News Service reported.
Heavy rains across much of southern China over more than a week have killed at least 199 people and left 123 missing, as rivers broke their banks and landslides severed road and rail links, China's civil affairs ministry said.
More than 2.38 million people have had to flee their homes.
The National Meteorological Centre has warned that the current rains could keep pounding parts of the south until Saturday.
The worst hit were the provinces and regions of Fujian, Jiangxi, Hunan, Guangdong, Sichuan, Guizhou and Guangxi.
The floods have caused economic losses estimated at around $6.2bn, with over 1.6 million hectares of farmland flooded and about 195,000 houses collapsing.