Boundaries

This ecoregion encompasses a transition zone between the Amazon lowlands and the Andes in the western Amazon along the Ucayali/Urubamba drainage. It comprises portions of the lower and upper Ucayali, Aguaytia, Pachitea, Tamaya, Urubamba, Perené, Ene, Tambo, and Apurímac basins between 250 and 750 m elevation.

Topography

This ecoregion lies at the eastern base of the Cordillera Oriental between 250 to 750 m asl. It includes two mountain ranges – the Cordillera Azul to the west and Serra do Divisor to the east – that flank the Río Ucayali.

Freshwater habitats

The Ucayali and its tributaries form the main headwaters of the Amazon River. After exiting the Andes, they meander over a shallower gradient and the flows become larger. Swollen from rains and snowmelt from the Andes, these whitewater rivers carry large sediment loads to the extensive várzea floodplains along the meandering lower main stem of the Ucayali. Most of the Ucayali’s sediment load comes from elevations below 500 m. The flood season lasts from December to May, with the highest water levels occuring in March.

Terrestrial habitats

The dominant vegetation types include Ucayali moist forests on the western side of the Ucayali and Southwest Amazon moist forests to the east, and range from lowland rainforest to pre-montane moist forests. Separating the two forested areas is Iquitos várzea that stretches along the floodplain of the Río Ucayali to the confluence with the Río Marañon.

Description of endemic fishes

There is only one endemic genus (Fonchiloricaria), although there are nearly 21 endemic species, including two species of Attonitus, three species of Bryconamericus, and four species of Creagrutus, all of which are typical of transitional piedmont regions.

Ecological phenomena

The piraíba (Brachyplatystoma filamentosum) and jaú (Zungaro zungaro) migrate as far west as this ecoregion to spawn.

Justification for delineation

This ecoregion falls in a transitional zone between the Guyanan-Amazonian ichthyographic region and Andean ichthyographic region (Gery 1969; Ringuelet 1975).

Level of taxonomic exploration

Fair to good in larger rivers, still poor in headwaters.