or Guainía River
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.