It joins the Amazon river 1400 km (870 miles) by river above Paraguay. The junction of the great river Beni with the Madeira is at the Madeira Fall, a vast and grand display of reefs, whirlpools and boiling torrents. Between Guajara-Merim and this fall, inclusive, the Madeira receives the drainage of the northeastern slopes of the Andes, from Santa Cruz de la Sierra to Cuzco, the whole of the south-western slope of Brazilian Matto (irosso and the northern one of the Chiquitos sierras, an area about equal to that of France and Spain. 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.
All of the upper branches of the river Madeira find their way to the falls across the open, almost level Mojos and Beni plains, 90,000 km2 (35,000 square miles) of which are yearly flooded to an average depth of about 3 feet for a period of from three to four months.
A subspecies of Boto (Amazon River Dolphin) is known to inhabit the Madeira river system.