This annual phenomenon forms the most extensive system of riverine flooded forests on Earth - a drastic revolution in the landscape that is vital for the efficient functioning of the Amazon River Basin.
Floodplain forests represent between 3-4% of the Amazon Basin area and are highly productive riverside areas that are flooded during the rainy season, and which receive rich sediment from the Andes Mountains. These factors have resulted in the evolution of ecosystems and habitats with a high number of species. They have also lead to a thriving economic activity of riverine communities.
There are three types of flooded forests; Varzea forests which are feb by muddy rivers, Igapo forests located in blackwater and clearwater tributaries, and tidal forest located in the estuary.
Why does flooding occur?Heavy seasonal rainfall, concentrated in the Eastern Andes and the Northwest area of the Basin, are responsible for the river level fluctuations throughout the year. Because of the magnitude of the Amazon Basin and the uneven distribution of the seasonal rainfall, different parts of the river system inundate at different times and the overall floods last longer than would be the case if temporal distribution of precipitation were the same throughout the basin.
Increased rainfall translates into increase discharge of the rivers. Because the Amazon Basin is mostly flat and there is more water than the waterways can contain, the water spills outside the riverbanks and into the low-lying floodplains. As the water spills over from the rivers, new bodies of water are created, such as ponds and oxbow lakes.
Flooded areas extend about 20 km from the river banks2 and during the wet season, these may rise between 7.5 m and 15 m.
Where flooding happensBecause of its vast area, not all parts of the Amazon Basin are subject to flooding at the same time. Generally, flooding occurs in the northern Amazon Basin (above the Equator) while floods are receding in the basin's southern part (below the Equator). Other parts can even experience two floods per year, which are different in intensity.
As the Amazon Basin floodwaters drain into the Atlantic Ocean, the water levels begin to fall and the forests "rise" again from the water. Wildlife reclaims its boundaries, and intense heat returns.
The importance of the Amazon floodplain forestFlooding radically alters the forest landscape and is vital in dispersing sediment and in fertilizing the varzea. Heavy silt loads, primarily from the Andes, are a major factor in enriching soil for cultivation.
As the waters creep over the land, the habitats that are created make it possible for aquatic organisms to navigate gallery forests (found alongside the riverbanks) and to seek food there.
Tiny organisms called zooplankton are consumed by fish and reproduction rate is highest during high water. When the flood recedes, zooplankton is carried by the flow and is distributed to many rivers, providing a much needed food base for a range of aquatic wildlife.3
Floodplain forests, subject of unwanted attentionFloodplain forests are among the most threatened of all ecosystems in South America due to logging and forest clearing. Intensive logging and selective exploitation of the kapok tree (Ceiba pentandra) and virola (Virola surinamensis) are accelerating deforestation. In Peru, it is oil exploitaition activities that pose the gravest threat to floodplain forests.
Varzea areas are threatened by cattle ranching and the increase in the number of introduced water buffalo. In addition, over-exploitation of fisheries to supply growing urban centres in the region has the potential to cause the extinction of prized species and the loss of subsistence for traditional riverine dwellers.
The sustainability of these fisheries is tremendously important for the entire population of the Amazon, for whom fish are an essential part of their diet and culture.
Moreover, badly planned damming could profoundly alter the beneficial aspects of flooding, disrupting many crucial ecological relationships.4
1Meggars, 1998 in Kricher, 1997
2Goulding, 1993 in Kricher, 1997
4Goulding et al, 1996 in Kricher, 1997