Where it meets the Paraná de Sapucuá it takes the name of lower Trombetas, and reaches up to 1.8 km (1.1 mi) in width, with the stream divided by several long and narrow islands.

[3] The river basin has an area of about 133,630 km2 (51,590 sq mi), with an intricate pattern of tributaries including the Poana, Anamu, Turuna, Inhabu, Mapuera and Paru de Oeste.

In the Saracá-Taquera National Forest the main streams in the Trombetas basin are the Papagaio, Água Fria, Moura, Jamari, Ajará, Terra Preta and Saracá.

The inferior zone of the river, as far up as the first fall, the Porteira, has but little broken water and is low and swampy; but above the long series of cataracts and rapids the character and aspect of the valley completely change, and the climate is much better.

[5] The river basin lies partly within the 4,245,819 hectares (10,491,650 acres) Grão-Pará Ecological Station, the largest fully protected tropical forest conservation unit on the planet.