River, northwestern South America. A major tributary of the Amazon River, it rises in the rainforest of eastern Colombia, where it is known as the Guainía, and forms a section of the border between Colombia and Venezuela.
It crosses Brazil and joins the Amazon at Manaus. It is about 1,400 mi (2,250 km) long and is a major transport artery. Its name comes from its jet-black colour, which is caused by the decomposition of organic matter and its low silt content.
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