Guanabara Bay is the oceanic bay in Rio de Janeiro state, south-east Brazil, with the city of Rio de Janeiro and Duque de Caxias on its western shore and Niterói and São Gonçalo on its eastern shore. It covers 384 km², which makes it Brazil's second-largest bay after the All Saints' Bay. It is 31 km long and up to 28 km wide, with a perimeter of 143 km, and it meets the Atlantic through a mouth just 1.5 km wide between Sugarloaf Mountain and the Pico do Papagaio. Millions of people live around its shores, and as of 2014 more than 70% of the sewage from Rio's 12 million inhabitants reached the bay untreated, and it has suffered three major oil spills. Against that, the Guapimirim Environmental Protection Area (1984) and the Guanabara Ecological Station (2006) protect its last large mangroves, and Sugarloaf, at the entrance, became part of a UNESCO World Heritage Site in 2012. This guide goes through the threats and the protections in date order.
From 1502 to 1975: How a Harbour Filled Up With City
Portuguese navigators under Gaspar de Lemos and Gonçalo Coelho first encountered the bay on 1 January 1502, and it was given the name Rio de Janeiro, "January River". The name Guanabara comes from the Tupi goanã-pará, usually glossed as "bay similar to the sea". Tamoio and Tupiniquim peoples lived on its shores. After French Huguenots under Nicolas Durand de Villegaignon invaded in 1555, briefly settling on Lajes Island and then Serigipe Island, and were expelled in 1563, the Portuguese ringed the narrow entrance with the forts of Santa Cruz, São João, Lajes and Villegaignon.
Land reclamation and building reshaped the shore. Downtown Rio's Santos Dumont Airport stands on reclaimed land. Flamengo Park, also called the Aterro do Flamengo, a 120-hectare park along the bay designed by Affonso Eduardo Reidy with gardens by Roberto Burle Marx, was completed in 1965. The Federal University of Rio de Janeiro's main campus sits on Fundão, an artificial island joined by bridges to Governador Island, home of Galeão International Airport. More than 130 islands still dot the bay. On 4 March 1974 the Rio–Niterói Bridge opened across it, 13.29 km long with 8.84 km over water and a central span 72 m high so ships can pass beneath.
Politics followed the shoreline. When Brazil's capital moved to Brasília on 21 April 1960, the old Federal District became the city-state of Guanabara, named after the bay. In 1975 the military government merged Guanabara with the surrounding state of Rio de Janeiro, and its territory became the municipality of Rio.
Sewage, Jardim Gramacho and the 2016 Olympic Sailing Course
Raw sewage is the first threat. The bay is the final receiver of all the rivers and streams that drain its shores and surrounding basins, and in 2014 more than 70% of the sewage from Rio's roughly 12 million inhabitants still flowed in untreated. Rivers such as the Meriti, which separates Duque de Caxias and São João de Meriti from the city of Rio, carry that load to the shore. Domestic and industrial discharges have also left oils and heavy metals in its sediments.
Solid waste added to it. From the 1970s, one of the world's largest landfills grew on an ecologically sensitive wetland at Jardim Gramacho in Duque de Caxias, right beside the bay. It closed in June 2012 after 34 years. It supported about 1,700 people who scavenged it for recyclables, and the 2010 film Waste Land documented the artist Vik Muniz working with them.
The bay came under world scrutiny when Rio won the 2016 Olympics on 2 October 2009. The host city promised to clean the water, and the sailing events were scheduled for 8–18 August 2016 from the Marina da Glória inside the bay. The promise was not fully kept. Scientists found drug-resistant "super bacteria" in the bay, linked to hospital waste and raw sewage, and the Oswaldo Cruz Foundation found the same genes in a river that empties into it. World Sailing examined holding the racing fully outside the bay or even moving it to Búzios, but the regatta stayed. The German sailor Erik Heil was infected with multi-resistant germs at a test event. In June 2014 the Dutch Olympic windsurfing champion Dorian van Rijsselberghe appealed for a clean-up, and the Netherlands put together a Clean Urban Delta Initiative for the bay with Dutch firms and research bodies. Critics warned that, without political incentive after the Games, the improvements would not last.
REDUC and the January 2000 Pipeline Spill
Oil is the second great threat. The bay shelters the Port of Rio de Janeiro, Brazil's tenth-busiest public port by cargo and its main cruise port. Its Caju Wharf alone has 24 storage tanks, 12,000 m³ of them reserved for petroleum products. On the northern shore at Duque de Caxias stands REDUC, the refinery operated by Petrobras.
There have been three major oil spills in the bay, in 1975, 1997 and 2000. The most recent came in January 2000, when a leaking Petrobras pipeline from REDUC released about 1.3 million litres of oil. Fish and crabs died, fishing in the bay came to a halt, and large areas of mangrove were killed. More than a decade later they had still not returned to life. The spill was a catalyst for new environmental law in Brazil, but the damage shows how slowly a polluted mangrove recovers once the trees die.
Guiana Dolphins: From 400 in 1985 to Fewer Than 30 in 2024
The clearest record of the bay's decline is its dolphins. Guiana dolphins (Sotalia guianensis) are small grey dolphins up to 2.1 m long that live in estuaries and sheltered shallows along South America's Atlantic coast. Individuals tend to stay in or return to the same area for years, which leaves a resident group like Guanabara's exposed when its home water deteriorates.
The Guanabara population fell from about 400 in 1985 to 70 in 1995 and 34 in 2016, and by 2024 it was below 30. Industrial waste and sewage are a possible cause. Neighbouring Sepetiba Bay and Ilha Grande Bay to the west hold larger groups, though they too are considered at risk, and Sepetiba Bay receives untreated sewage from a population of around 1.29 million. The great whales have largely gone too. The bay was once a whaling ground, but whales are now rarely seen inside it, although Bryde's whales can still be seen around the entrance.
Guapimirim 1984 and Guanabara Ecological Station 2006: The Last Mangroves
The main legal protection lies in the bay's quiet north-eastern corner, where the Guapi-Macacu, Macacu/Caceribu and Guaxindiba rivers spread silt across tidal flats. The area was considered naval land that could not be occupied, but damage continued. Scientists and local community representatives campaigned together for protection, and on 25 September 1984 presidential decree 90.225 created the Guapimirim Environmental Protection Area. It covers 13,926 hectares across the municipalities of Guapimirim, Itaboraí, Magé and São Gonçalo. It is an IUCN category V protected landscape, meant both to save the remaining mangroves and to sustain the traditional communities that live closely with them.
The core was tightened on 15 February 2006, when a federal decree created the Guanabara Ecological Station, a strictly protected reserve of 1,936 hectares that lies entirely inside the APA. Both are run together by the Chico Mendes Institute for Biodiversity Conservation, and the station covers the best-preserved part of the bay. Its mangroves are mainly red mangrove (Rhizophora mangle), Avicennia schaueriana and Laguncularia racemosa. They shelter species threatened in Rio de Janeiro state, including the anhinga, the fulvous whistling duck and the broad-snouted caiman, along with migrating ospreys and semipalmated plovers. Where people once cleared the forest, invasive plants such as Hibiscus pernambucensis, the fern Acrostichum aureum and cattails (Typha) have taken hold instead. For how these coastal forests work elsewhere, see our global guide to mangrove forests.
Sugarloaf, UNESCO 2012 and the Serra dos Órgãos: Protection Around the Rim
The mountains behind the bay are protected too. On 30 November 1939 Getúlio Vargas's government created the Serra dos Órgãos National Park, Brazil's third national park, to protect the range's peaks and the headwaters of the rivers flowing into the Fluminense basin. The park's lowest point, at 145 m, lies in Magé, a municipality on the bay's northern shore, and a 1984 decree fixed its area at 10,527 hectares. Its highest point, Pedra do Sino, rises to 2,263 m, and its Dedo de Deus peak appears on the flag of Rio de Janeiro state. Closer to the city, protection of the Tijuca forest began with an 1817 decree of King João VI banning tree-cutting around the springs of the Carioca River to safeguard Rio's water supply. Today Tijuca National Park is part of the Atlantic Forest Biosphere Reserve.
At the entrance, Sugarloaf Mountain, a granite peak rising 396 m above the harbour, has been protected since 2006 as part of the Sugarloaf Mountain and Urca Hill Natural Monument. In 2012 it became part of a UNESCO World Heritage Site covering the landscapes around Rio, Tijuca National Park included. Those listings protect the view at the bay's mouth. The water inside it, fed by rivers like the Meriti, depends on sewage treatment and pollution controls that have so far fallen short of their targets.
Seen from Sugarloaf on a clear day, the bay keeps the shape the Portuguese saw in 1502, even with a city of millions around it. If you are planning to see it in person, our guide to the best time to visit Rio de Janeiro covers the seasons around the bay.