The Tocantins is a river, the central fluvial artery of Brazil. Running from south to north for a distance of about 1500 miles, it is not really a branch of the Amazon River, although usually so considered, since its waters flow into the Atlantic Ocean alongside those of the Amazon.

It rises in the mountainous district known as the Pyreneos; but its more ambitious western affluent, the Araguay, has its extreme southern headwaters on the slopes of the Serra Cayapo, and flows a distance of 1080 miles before its junction with the parent stream, which it appears almost to equal in volume. Besides its main tributary, the Rio das Mortes, it has twenty smaller branches, offering many miles of canoe navigation. In finding its way to the lowlands, it breaks frequently into waterfalls and rapids, or winds violently through rocky gorges, until, at a point about 100 miles above its junction with the Tocantins, it saws its way across a rocky dyke for 12 miles in roaring cataracts.

The tributaries of the Tocantins, called the Maranhao and Parana-tinga, collect an immense volume of water from the highlands which surround them, especially on the south and south-east. Between the latter and the confluence with the Araguay, the Tocantins is occasionally obstructed by rocky barriers which cross it almost at a right angle. Through these, the river carves its channel, broken into cataracts and rapids, or cachoeiras, as they are called throughout Brazil. Its lowest one, the Itaboca cataract, is about 130 miles above its estuarine port of Cameta, for which distance the river is navigable; but above that it is useless as a commercial avenue, except for laborious and very costly transportation.

The flat, broad valleys, composed of sand and clay, of both the Tocantins and its Araguay branch are overlooked by steep bluffs. They are the margins of the great sandstone plateaus, from 1000 to 2000 foot elevation above sea-level, through which the rivers have eroded their deep beds. Around the estuary of the Tocantins the great plateau has disappeared, to give place to a part of the forest-covered, half submerged alluvial plain, which extends far to the north-east and west. The Para River, generally called one of the mouths of the Amazon, is only the lower reach of the Tocantins. If any portion of the waters of the Amazon runs round the southern side of the large island of Marajo into the river Para, it is only through tortuous, natural canals, which are in no sense outflow channels of the Amazon.