A section of a river flowed red due to excessive levels of iron ion and industrial waste in a town in north China's Hebei Province, local authorities announced Thursday.

The 4-km stretch of the Zhongting River in Shengfang Town, Bazhou City, has run red in recent days due to iron ions and rising temperatures, said a Bazhou government official.

Untreated industrial sewage, especially from local iron and steel plants, seriously polluted the waterway about a decade ago. Since 2005, however, the city has treated the river, but iron ions remain in the silt, said the official.

Currently, the water in the river comes from rainfall and treated industrial sewage, said the government.

The river is a flood control waterway that passes the cities of Langfang and Bazhou before emptying into the Haihe River in Tianjin Municipality. It is not a drinking-water source. It was dammed several years ago at the border area between Bazhou and Tianjin to prevent water flowing into Tianjin.

Environmental authorities will carry out regular water quality inspections and ensure industrial waste is discharged after being treated, said the official.

Experts have collected samples of the silt for further tests to help with treatment plans, said the official.