The foam is caused mostly by untreated household runoff from nearby Sao Paulo.
Mata Atlantica, an NGO and activist group that has been pushing for the clean-up of the river for years, said on Wednesday that the government has fallen behind in its plan to restore and clean up the river.
Malu Ribeiro, a spokesperson for the group, says the energy company controlling upriver dams opened the gates after rain caused a rise in reservoir waters.
“This is the result of an environmental crime, caused on 29 August, when the hydroelectric dams located in the metropolitan areas of São Paulo and the interior were opened,” Ms Ribeiro said.
Ms Ribero claims the company did not consider the accumulation of contaminated sediment at the bottom of the dam gates, which occurred after months of drought when the gates were closed due to low water levels.
The sudden release of water brought with it tons of chemicals and wastewater that had accumulated over months, leaving a toxic white foam on the surface of the river, emitting a sulphurous cloud and killing thousands of fish.
Tiete River has a length of 1150 kilometres and a basin area of 150 thousand square kilometres. It runs along the coast through São Paulo and dozens of smaller municipalities.
– Reported with Associated Press