Oiapoque

(also Oyapock), a river in South America that forms part of the border between Brazil and French Guiana. Measuring 485 km long, the river originates in the Tumuc-Humac (Tumucumaque) Mountains and empties into the Atlantic Ocean. It flows through the crystalline Guiana Highlands, forming a great number of rapids and waterfalls. The navigable lower Oiapoque crosses the coastal lowland.