Madeira mədā´rə [key], river, c.2,100 mi (3,380 km) long, formed by the junction of the Beni and Mamoré rivers on the Bolivia-Brazil border. It flows north along the border for c.60 mi (100 km), then northeast in a winding course through the Rondônia and Amazonas sections of NW Brazil into the Amazon River. At its mouth is Ilha Tupinambaranas, an extensive marshy region formed by the Madeira's distributaries. The river receives numerous tributaries from the southeast and is navigable by ocean vessels to Pôrto Velho in Rondônia; the falls and rapids above the city are now submerged by the Santo Antonio dam and hydroelectric project. The former Madeira-Mamoré RR ran 227 mi (365 km) around the unnavigable section to Guajará-Mirim on the Mamoré River. A second Brazilian hydroelectric project, associated with the Jirau dam, is c.60 mi (100 km) upstream from the Santo Antonio dam.

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