Environmental authorities in northwest China's Shaanxi Province said Wednesday that they will start a pollution control overhaul on plants along the Weihe River, the biggest and most polluted tributary of the Yellow River.

Authorities will check pollution control operations for 86 companies, 409 animal farms and 39 sewage treatment plants in the Weihe River basin, said Li Xiaolian, deputy head of the province's environmental protection department.

Li said that the overhaul will start in February and last through March.

The 818-km Weihe River, the longest tributary of the Yellow River, originates in Gansu province and flows past major industrial cities in Shaanxi, including Baoji, Xianyang, Xi'an and Weinan.

The river has been seriously polluted over the past two decades as a result of waste discharges from paper mills and other high-polluting plants. Despite years of efforts to curb pollution, the Weihe River remains the Yellow River's most polluted tributary.