Tapajós River, a principal tributary of the Amazon River. The Tapajós is formed by the junction of the Juruena and Teles Pires (Sāo Manuel) rivers in north-central Brazil and flows some 500 miles (800 km) northeastward to the Amazon at Santarém. The total length, including the Juruena and its longest headstream, the Arinos, is about 1,250 miles (2,000 km). Only 150 miles (240 km) are navigable.