Madeira (mədāˈrə) [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 the falls and rapids near Pôrto Velho, Brazil. There the Madeira-Mamoré RR begins a 227 mi (365 km) run around the unnavigable section to Guajará-Mirim on the Mamoré River.