Definitions

hidro-

Tocantins River

[taw-kahn-teens]

The Tocantins is a river, the central fluvial artery of Brazil. It runs from south to north for 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 flows through four Brazilian states (Goiás, Tocantins, Maranhão and Pará) and gives its name to one of Brazil's newest states, formed in 1988 from what was until then the northern portion of Goiás.

It rises in the mountainous district known as the Pireneus, west of the Federal District, but its western tributary, the Araguaia River, has its extreme southern headwaters on the slopes of the Serra dos Caiapós. The Araguaia flows 1080 miles before its confluence with the Tocantins, to which it is almost equal in volume. Besides its main tributary, the Rio das Mortes, the Araguaia 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.

Two other tributaries, called the Maranhão and Paranatinga, 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 Araguaia, the Tocantins is occasionally obstructed by rocky barriers which cross it almost at a right angle.

Downstream from the Araguaia confluence, in the state of Pará, the river used to have many cataracts and rapids, but they were flooded in the early 1980s by the artificial lake created by the Tucuruí dam, one of the world's largest. When the second phase of the Tucuruí project is completed, there will be a system of locks that will make a long extension of the river navigable. The construction works on the locks have been stalled for many years due to lack of funding, but it is possible that they will be included in a massive development program launched by the Brazilian government in 2007, in which case they could be operational within about four years.

The flat, broad valleys, composed of sand and clay, of both the Tocantins and its Araguaia 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 Pará 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 Marajó into the river Para, it is only through tortuous, natural canals, which are in no sense outflow channels of the Amazon.

The Tocantins River records a mean discharge rate of 13,598 m³/s (9% of the national total) and a specific discharge rate of 14.4 l/s/km². The sub-basins have the following specific discharge rates: Tocantins (11 l/s/km²), Araguaia (16 l/s/km²), Pará (17l/s/km²) and Guamá (21l/s/km²).

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