The first includes the Guapay or Rio Grande, Piray or Sara, Yapacani and Marac6, upper tributaries of the Mamore, and the San Miguel, Blanco, Baures and Paragua, tributaries of the Guapore - both draining the western and northern parts of the department.
Its Brazilian tributaries are comparatively unimportant, but from Bolivia it receives the Baures and the San Miguel, both rising in the Sierras de Chiquitos and flowing north-west across the llanos to the Guapore.
The waters find their way to the falls of the Madeira by many great rivers, the principal of which, if we enumerate them from east to west, are the Guapore or Itenez, the Baures and Blanco, the Itonama or San Miguel, the Mamore, Beni, and Mayutata or Madre de Dios, all of which are reinforced by numerous secondary but powerful affluents.
San Antonio is the first of a formidable series of cataracts and rapids, nineteen in number, which, for a river distance of 263 m., obstruct the upper course of the Madeira until the last rapid called Guajara Merim (or Small Pebble), is reached, a little below the union of the Guapore with the Mamore.
The Guapore, or Itenez, an affluent of the Mamore, is the third large river of this Bolivian drainage basin, but it rises in Brazil, on the south slopes of the Sierra dos Parecis, where it flows in a great bend to the south and then west of north to the Bolivian frontier in 14° S.