a river in South America, a left tributary of the Amazon. The upper Putumayo forms the border between Colombia and Ecuador, and the middle Putumayo forms the border between Colombia and Peru. In its lower course the river flows through Brazil, where it is called the Içá. The Putumayo is 1,580 km long and drains an area of 123,000 sq km. It originates in the southern Andes of Colombia and flows primarily through the humid equatorial forests of the Amazon Basin. The mean annual flow rate of the Putumayo is 7,160 cu m per sec. The river is fed by rain. High water is from April through July. The lower course of the Putumayo is navigable.