river, chiefly in western Brazil, rising on the Peruvian border, along which it continues eastward to form part of the Brazil–Bolivia border. Turning north at Brasiléia, the remainder of its 400-mi (645-km) course flows in a north-northeasterly direction, through the Brazilian states of Acre and Amazonas, to join the Rio Purus, a tributary of the Amazon, at Bôca do Acre. It was formerly called the Aquiri.