People living in six cities nearby have been told not to drink or bathe in river water.
Hundreds of fish, cattle and other animals that drank from the river have died.
It's happened in Minas Gerais state in southeastern Brazil, 200km north of the capital Rio de Janeiro.
A reservoir in Cataguazes that stored chemicals produced at an abandoned factory burst.
May take years for rivers to recover
Caustic soda is a very poisonous and harmful chemical.
The environmental authorities there are now trying to work out how much of it leaked into the rivers.
Oil workers have been brought in to try and stop any more damage being done.
One Brazilian journalist reckons it'll take 10 years for the river to recover.
The company responsible for the reservoir has been fined £10 million.
Hear story
Chat about the environment
I want to make the Earth better
