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Guaporé (gwəpŏŏrĕ´), river, c.750 mi (1,207 km) long, rising in the mountains of Mato Grosso state, W Brazil. It flows northwest through rain forest and forms part of the Brazil-Bolivia border before joining the Mamoré River. The river is also called Iténez. Guaporé was the original name (until 1956) of the Brazilian federal territory that is now the state of Rondônia.