On 5 November, a huge mudflow contaminated with iron ore from mine workings was released into the Rio Doce river in southeast Brazil after two dams broke. Immediate action is necessary to evaluate the massive human and ecological impact of this catastrophe, and there must be a concerted effort to prevent further such incidents.

As well as killing several people, the accident threatens the water supply of many large cities downstream that are already severely limited by a long-standing drought. The polluted river runs through the Atlantic rainforest and is likely to damage the exceptional endemic fauna and flora in its waterways.

Of the 71 recognized fish species in the river, 11 were considered endangered before the mud slide (see go.nature.com/zmry1z; in Portuguese). The accident also interrupted reproductive migrations for many of these species.