Which river forms the border between Brazil and French Guiana?

The Oyapock River forms most of the border between Brazil and French Guiana.

The river runs through the Amazonian region of South America, separating Brazil’s Amapá state from French Guiana, an overseas department of France. It rises in the Tumuk Humak Mountains and flows roughly 403 kilometres before reaching the Atlantic Ocean.

The boundary has a complicated history. The 1713 Treaty of Utrecht referred to a river called the Japoc, but France and Brazil disagreed over whether that name meant the Oyapock or the farther-south Araguari River. The dispute was settled in Brazil’s favour by Swiss international arbitration in 1900.

A common mix-up is to answer the Amazon, which is much larger but does not form this international boundary. The Oyapock River Bridge, opened to the public in March 2017, links Oiapoque in Brazil with Saint-Georges-de-l’Oyapock in French Guiana. In Portuguese, the river is commonly called the Oiapoque.