tributary, 1,000 miles (1,609 km) long, of the Amazon River. It originates as the Guamués River, which flows from La Cocha Lake, high in the Andes near Pasto, Colombia. The Guamués flows southeastward into densely forested plains past Puerto Asís, Colom., after which point it is known as the Putumayo.

The river continues its southeasterly course through tropical rain forest, forming most of the border between Colombia and Ecuador and Colombia and Peru. At Santa Clara, Colom., it enters Brazil, where it is known as the Içá. The river joins the Amazon at Santo Antônio do Içá, Brazil.

The Putumayo is a major transportation artery, navigable for almost its total length. On its banks are numerous small river ports, and rubber is gathered from forests along its course.