A battle of forces
The mouth of the Rio Doce is a dynamic area, shaped by a constant tug-of-war between the river and the Atlantic Ocean. The river's discharge pushes freshwater and sediment out to sea, while the ocean's waves and longshore currents push sand along the coast, primarily from south to north. When the river's flow is strong, particularly during flood seasons, it keeps the channel open. However, when the river's discharge lessens, the persistent force of the ocean can build a sandbar across the mouth, sometimes sealing it completely.
This process creates a temporary lagoon, dramatically altering the local environment. Studies of the area's morphology show significant changes over short timescales. For instance, between 2017 and 2022, the width of the river mouth fluctuated dramatically. In 2017, a large central sand bar left only two narrow channels, totaling about 153 meters in width, for the river to meet the sea. By 2022, these bars had shifted, opening the mouth to a width of 684 meters. This cycle of closing and opening is a natural geological cycle, but one that has been greatly affected by human activity.
An ecosystem under pressure
On November 5, 2015, the Fundão dam, an iron ore tailings dam, collapsed over 600 kilometers upstream. This event, considered Brazil's worst environmental disaster, released an estimated 50 to 60 million cubic meters of mining waste into the Rio Doce. The toxic sludge traveled the length of the river and reached the Atlantic Ocean sixteen days later, drastically altering the sediment composition at the mouth.
The influx of fine, metal-laden sediment has had lasting consequences. The waste smothered the riverbed and coastal shelf, impacting the benthic macrofauna—organisms living in the sediment that form the base of the local food web. The disaster devastated local fishing communities in villages like Regência, which depend on species like the snook (robalo), croaker (corvina), and various catfish (barbudinho). The closure of the river mouth, now complicated by the contaminated sediment, traps fish and disrupts the migratory paths important for their spawning cycles. Fishing in the estuary was banned by court order following the disaster, compounding the economic hardship for locals. The long-term effects of the tailings, which contain elevated levels of arsenic, lead, and other metals, continue to be a subject of scientific study.