Madeira, Or Madera, (Port and Span., wood), a river of South America, the largest of the affluents of the Amazon, formed by the united waters of the Beni and the Mamore or Grande, which drain almost three fourths of the Bolivian territory, and of the Guapore or Itenez, which flows through the auriferous plains of the Brazilian province of Matto Grosso. The Madeira proper begins at the confluence of the Mamore and the Itenez, about lat. 12° S.; it receives the Beni on the left bank in lat. 10°, flows N. E., and falls into the Amazon midway between Manaos and Serpa, about lat. 3° 30' S., after a course of 750 m.; but its entire course is reckoned at 2,000 m. At its mouth it is 2 m. wide and 65 ft. deep; 500 m. up its width is 1 m. and its depth 100 ft.; and were it not for a series of 12 magnificent cataracts commencing some 480 m. from the Amazon, vessels of almost any size might sail up into the very heart of Bolivia. But a railway of about 150 m., traversing the bend of the river in which the falls are comprised, which was commenced in 1872 by an American engineer, Col. E. Church, is to connect the navigation of the upper and lower Madeira. Traffic has hitherto been carried on by canoes 30 to 40 ft. long, which have to be lifted out of the water on nearing the successive cataracts.

The river derives its name from the vast quantity of wood that floats down its stream, especially during and for a short time after the floods.