river in northwestern Pará state, northern Brazil. Formed by the Poana, Anamu, and other headstreams flowing from the southern slope of the Serra Acaraí on the Guyana border, the Trombetas meanders generally southward for 470 mi (760 km). It forms several lakes, including Jamari and Erepecu, before joining the Amazon River upstream from Óbidos. Only the lower course of the Trombetas is navigable; elsewhere falls and rapids interrupt the river flow.