Nobody immediately gave a reason for the river turning red, but Wenzhou Environmental Protection Bureau inspectors have taken samples for analysis, reported China Radio International.
According to a local contacted by the station, nothing of the kind had ever happened before and there was no chemical plant upstream.
However, in September 2012, the Yangtze River mysteriously started running red. At that point people considered industrial pollution or silt being churned up as possible causes.
After a few images were reviewed, however, scientists suggested the cause could have been more man made.
Emily Stanley, a professor of limnology (the study of inland waters) at the University of Wisconsin told LiveScience at the time:
“It looks like a pollutant phenomenon,” she said. “Water bodies that have turned red very fast in the past have happened because people have dumped dyes into them.”