Boundaries

This ecoregion encompasses the mid-upper basins of the rivers draining the southern slopes of the Guiana Shield between the upper Rio Arapari in Amapá to the east and the Rio Urariquera to the west in Roraima.  It includes the upper Rio Branco basin upstream from Caracaraí and the upper portions of the Rio Demeni and Rio Padauiri. It is divided from the Orinoco Basin by the Serra Paraima and Serra Parima.

Topography

The rivers of this ecoregion drain the ancient Guiana Shield. The Tumucumaque Uplands in the north are undulating and mountainous, reaching elevations of 2100 m asl. These ancient crystalline uplands consist mostly of sandstone and quartzite. The Serra Pacaraima and Serra Parima are further northwest. The southern part of the ecoregion comprises low-lying plains of the Amazon Basin and reflect more recent alluvial deposits from lacustrine and marine environments.

Freshwater habitats

Deep valleys with fast-flowing rivers, such as the Rio Urariquera (Uraricoera), characterize the northwestern part of the ecoregion. The Rio Branco (mean annual discharge, 2900 m³/s) is unique in that it is a whitewater river surrounded by blackwater and clearwater rivers, the largest of which are the rios Jari and Trombetas. The blackwater and clearwater rivers draining the Guiana Shield have low levels of suspended sediments and nutrients. Even though the Branco is considered whitewater, it has very low electrical conductivity (20-30 µS) and is less muddy than that of the Amazon since the Guiana Shield, from which it originates, contains older and more chemically eroded soils, resulting in less turbidity than rivers draining the Andes.

The Trombetas is the second smallest of the tributary basins in the Amazon. Originating in the Tumucumaque Uplands on the Guiana Shield, it is a clearwater river with many cataracts along its course. The lower Trombetas has a large floodplain with many lakes. The flood season lasts from March to August. There is also an intense dry season between September and February, causing many of tributaries in the mid and upper Trombetas basin to reduce their flow drastically.

Terrestrial habitats

Lowland moist forests cover the southern part of the ecoregion, with common families including Sapotaceae, Lecythidaceae, Burseraceae, Fabaceae, Rubiaceae, Chrysobalanaceae, Lauraceae, Annonaceae, Moraceae, Mimosaceae, and Caesalpinaceae. The banks of the lower Rio Branco are covered by extensive várzeas, whereas igapó forests blanket the banks of many of the other rivers in the ecoregion. North of Caracaraí the habitat changes from moist lowland forests to open grass savanna locally called the Lavrado, which is riddled with permanent and seasonal lagoons and gallery forests dominanted by Mauritia palms. Within the lowland forests are large areas of campinarana (caatinga) that form a patchwork of vegetation types from herbaceous savannas to closed-canopy forests. These occur on sandy, oligotrophic soils around circular swampy depressions. Some of the species limited to campinarana include Virola parvifolia, Compsoneura debilis, and Pithecelobium leucophyllum, among others. 

Description of endemic fishes

Currently there are nearly 40 endemic species in 12 families recognized in this ecoregion. There are no endemic genera. Some genera with multiple endemic species include Hypostomus and Harttia (Loricariidae), Hyphessobrycon (Characidae), and Apistogramma and Crenicichla (Cichlidae).

Other noteworthy fishes

Species of several genera of Pimelodids catfishes (Zungaro, Phractocephalus, Brachyplatystoma, Leiarius) are abundant in the deep pools and reach very large sizes. Very large specimens of Hydrolycus armatus and Hoplias aimara are also known to occur in these rivers. Chaetostoma jegui is endemic to the region and is the only representative of the genus that occurs outside of the Andean forefront.

Ecological phenomena

The piraíba (Brachyplatystoma filamentosum) and jaú (Zungaro zungaro), known for their spawning migrations, occur in this ecoregion. The ecoregion also contains the potamadromous barred sorubim (Pseudoplatystoma fasciatum) and P. punctifer.

Justification for delineation

This ecoregion falls within the Guyanan-Amazonian ichthyographic region, and more specifically within the Amazonian ichthyographic province (Gery 1969; Ringuelet 1975). It contains a unique assemblage of Guiana Shield species, distinct from all other fauna within the Amazon Basin.

Level of taxonomic exploration

Fair in large rivers, but remains poor in the headwaters.