In this region the whitewater Amazon is fed by the blackwaterXingú, Jari, and Tapajós rivers. Elevations are no more than. The Gurupa várzea contains floodplains along the river that are affected by daily Atlantic Ocean tidal fluctuations and seasonal flooding. At the height of the rainy season when the Amazon is in full flood water levels may fluctuate by in a tidal period as the river water is dammed by the rising ocean tide. Outside the rainy season fluctuations may still be from in a tidal period. The region contains a complex maze of channels and sedimentary islands. The soil is made up of sediments carried from the eastern Andes by the rivers during the present Holocene epoch. The river sediments form levees along the margin of the river, behind which are areas of savanna on clay soil and open lakes. Both the savanna and the lakes flood daily, and the lakes grow much larger at the height of the flood season.
The Köppen climate classification is "Am": equatorial, monsoon rainfall. Temperatures are fairly steady throughout the year, ranging from with an average of. Mean annual precipitation is about, with rainfall varying from in October to in April.
Flora
The levees hold tropical rainforest with a height of around. Behind them the ecoregion holds a distinctive form of várzea that is mostly savanna rather than dense rainforest. The clay soil supports robust savanna known as canarana. The rich sediments make the várzea very fertile compared to the terra firme land further from the river. The vegetation stabilizes the soil and is home to a very diverse population of aquatic mammals and freshwater fish. The flooded savanna holds large grasses such as Echinochloa polystachya, Echinochloa spectabilis, Hymenachne amplexicaulis, Hymenachne donacifolia, Leersia hexandra, Paspalum platyaxix, Luziola spruceana, Panicum elephantipes, Paspalum fasciculatum and several Oryza species. Some of the grasses also grow on former channels of the river. Among the grasses there are sedges such as Scirpus cubensis, Cyperus luzulae, Cyperus ferax and Scleria geniculata. The areas of transition between the forest and river hold shrubs and small vines that include Artemisia artemisiifolia, Ipomoea carnea, Polygonum punctatusm, Justicia obtusifolia, Alternanthera philoxeroides, Capironia fistulosa, Sesbania exasperata, Mimosa pigra, Montrichardia linifolia, Clamatis aculeata, Senna reticulata, Phaseolus lineatus, Rhabdadenia macrostoma and Clitonia triquetum. The forest holds larger trees such as Hura crepitans, Triplaris surinamensis, Calycophyllum spruceanum, Cedrela oderata, Pseudobombax munguba, Virola surinamensis and Ceiba pentandra. A variety of trees produce fleshy fruit that supports species of fruit eating primates, bats and fish that enter the forest when it is flooded. These include yellow mombim, buriti palm, açaí palm, socoró and tarumã.
As of 2017 the World Wildlife Fund classed the ecoregion as "Critical/Endangered". Human activities in the ecoregion include fishing, subsistence agriculture, selective logging and cattle or water buffalo ranching. The small-scale farmers typically practice diverse agriculture that depends on the forest and does not degrade the land. In the past, large scale production of jute damaged the environment, and mechanized rice production continues today. Elsewhere large areas of natural forest along the Amazon banks have been cleared and the savanna converted to pasturage. Most of the commercially value timber species such as Virola surinamensis and Ceiba pentandra have been harvested. The remaining forest is mostly secondary growth, either managed or unmanaged. Urban sprawl in the city of Monte Alegre has also affected the ecoregion.
