River of northwest Brazil; length 3,250 km/2,020 mi. It is the longest tributary of the Amazon, and is formed by the rivers Beni and Mamoré. It flows northeast through the state of Amazonas to join the Amazon east of the city of Manaus. It is navigable by ocean-going vessels for 1,144 km/711 miles as far as Pôrto Velho. Gold-prospecting still occurs near the Bolivian border.
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