As the outlet of the Tocantins is so near to that of the Amazon, and their lower valleys are conterminous, it is convenient to treat them as parts of the same hydrographic basin.
By the deeply eroded valleys of the Araguaya and Upper Tocantins rivers and their tributaries.
The second system - the Central or Goyana - consists of two distinct chains of mountains converging toward the north in the elevated chapadao between the Tocantins and Sao Francisco basins.
Although the northern and southern extremities of Goyaz lie within two great river systems - the Tocantins and Parana - the upper courses of which are navigable, both of them are obstructed by falls.
ARAGUAYA, ARAGUAY or Araguia, a river of Brazil and principal affluent of the Tocantins, rising in the Serra do Cayap6, where it is known as the Rio Grande, and flowing in a north by east direction to a junction with the Tocantins at Sao Joao do Araguaya, or Sao Joao das Duas Barras.
The Tocantins is sometimes treated as a tributary of the Amazon because its outlet, called the Rio Para, is connected with that great river by a number of inland channels.