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Mamoré (məmŏŏrĕ´), river, c.600 mi (965 km) long, formed by tributaries rising in the Andes and plains of central Bolivia. It flows north, past Trinidad, to the Brazilian border. After forming part of the Bolivia-Brazil border, the Mamoré joins with the Beni River to form the Madeira River. With the Río Grande, its chief tributary, the Mamoré flows c.1,100 mi (1,770 km) through the Bolivian lowlands (where it is navigable) and the Cordillera Oriental.