a river in western Brazil, a right tributary of the Amazon. Length, 3,280 km; basin area, about 200,000 sq km. The river rises in the foothills of the Peruvian Andes (La Montana). In its middle and lower reaches the river’s tortuous route cuts through the Amazon lowlands. Fed by rain, the water level is high from January to May and low from August to October. The average flow is 6,000 cu m per sec. The river is navigable below the city of Cruzeiro do Sul.