River, northern South America.
Rising in Venezuela and flowing through Guyana, it is about 350 mi (560 km) long and forms the border between Venezuela and Guyana for some 60 mi (100 km). It then flows southeast to join the Mazaruni River before entering the Essequibo River . Rapids impede navigation, but the river is important for its alluvial gold and diamonds. It was designated a UNESCO World Heritage site in 1983.