The várzea forests of this ecoregion extend along the low, seasonally flooded rivers of the central and lower basin of the Amazon River, including a large part of the Madeira River basin, the mouth of the Purus River, tributaries of these rivers and an isolated patch of várzea along the Mamoré River between Bolivia and Brazil.[1]
Major population centers in or near the ecoregion are Manaus, Itacoatiara, Coari and Óbidos.[2]
Elevations range from 15 metres (49 ft) in the east, where the Tapajós meets the Amazon river, to 80 metres (260 ft) on the Madeira.
The soils are fertile sediments formed in the present Holocene epoch, carried down from the Andes.
The annual floods renew the sediments, making very rich soils compared to the higher terra firme on either side of the várzea.[2]
The river waters, loaded with sediment, rise by 6 to 12 metres (20 to 39 ft) each year, flooding the land for as long as eight months.[1]
The river course through the floodplain constantly shifts over time, creating oxbow lakes, levees, meander swales and bars.
These landscape elements support diverse vegetation, predominantly seasonally flooded tropical evergreen rainforest.
Typically the levees are relatively high, formed by the deposit of sediments along the river margins.
Behind them the land slopes down, flattens out and then rises to the terra firme forest.
The area behind the levees holds canarana grassland and lakes that expand and contract with the flood cycle.[2]
Climate
The Köppen climate classification is "Af": equatorial, fully humid.
Average monthly temperatures range from 23 to 32 °C (73 to 90 °F) with an annual average of 28 °C (82 °F).
Average annual rainfall in the east is less than 2,000 millimetres (79 in), while in some parts of the Madeira in the west it exceeds 2,500 millimetres (98 in).[2]
The main vegetation types are aquatic vegetation, permanent swamp vegetation, succession vegetation and forest mosaics.
Typically there are fewer tree species in the várzea than on terra firme, although many trees are common to both environments.
Trees in the várzea are usually shorter than on terra firme, with a canopy of up to 25 metres (82 ft).
There are some palms, and often a dense understory of plants in the genus Heliconia and families Zingiberaceae and Marantaceae.
Tree species that grow only in wetlands include Virola surinamensis, Calycophyllum spruceanum and Açaí palm (Euterpe oleracea).
The economically valuable palms Astrocaryum jauari and Mauritia flexuosa are common on the floodplain.[2]
Other trees that feed the fruit-eating fish that enter the forest during flood periods are the yellow mombim (Spondias mombim), socoró (Mouriri ulei) and tarumã (Vitex cymosa).[2]
The World Wildlife Fund lists the ecoregion as "Critical/Endangered".
The ecoregion is threatened by contamination of the water and fish by gold mining, and heavy sedimentation caused by destruction of the forests. Huge areas of the floodplain forests are cleared and burned by ranchers to create pastures for livestock.[2]
Global warming will force tropical species to migrate uphill to find areas with suitable temperature and rainfall.
Low, flat, deforested ecoregions such as the Monte Alegre várzea are extremely vulnerable.[5]