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in its upper course called the Guayabero, a river in Colombia (South America), left-bank tributary of the Orinoco River. Length, about 1,300 km; basin area, about 140,000 sq km. It rises in the Eastern Cordillera, flows through the Mesa de Pardaos highlands, where it forms three falls, and then through the Orinoco lowlands. The river abounds in rapids; it is fed mainly by rain, and summer freshets are characteristic. It is navigable for small craft from the mouth of a left-bank tributary, the Ariari River (with interruptions at rapids).