Georg Irion

The large central Amazonian river floodplains near Manaus: geological, climatological, hydrological and geomorphological aspects

The Amazon River and its large tributaries are accompanied along their middle and lower courses by large fringing floodplains which cover an area of about 300 000 km2 (Sect. 1.5). Their local names are: “varzea” and “igapo”. These terms have been adopted in scientific literature, but until now they have not been defined. On the basis of hydrochemical investigations, Sioli (1950, 1951, 1956, 1965a, 1968) used the term varzea for floodplains along white-water rivers, which are rich in nutrients and suspended matter, and igapo for those along black-water rivers, which are poor in both. This nomenclature is supported by the zoological and botanical investigations of Irmler (1977) and Prance (1979), and it will be adopted here as well. The term “floodplain” will be used when referring to the general category. The periodically flooded varzea and igapo are contrasted with the upland that is not flooded, referred to as the “terra firme”. The transition between flooded and nonflooded areas can be very abrupt, being formed by steep bluffs as high as 100 m.

Quaternary geology of the Amazonian Lowland

The Quaternary history of the Amazon lowlands is characterized by deposition of sediments of Andean provenance and by the influences of changing sea levels. Areas well above the present water tables were not reached by Pleistocene high-water stages. These areas have been intensively weathered since the Tertiary, forming hard lateritic weathering horizons. These weathering horizons are best explained by the relatively constant, humid tropical climate throughout the Quaternary. In the western Amazonian Lowland, flood plains corresponding to the different Pleistocene sea-level heights were formed. During low sea level, erosion in the drainage areas increased and the water levels of the central Amazon River system were lowered. Valleys drowned and lakes formed in the lower reaches of rivers and creeks during high sea-level stages. These lakes (ria lakes) remained in the valleys with rivers having a low sediment load. Seismic profiling (3.5 kHz) in some of these lakes clearly showed deposits of the three last periods of Quaternary high sea-level stages.

Development of the Amazon Valley During the Middle to Late Quaternary: Sedimentological and Climatological Observations

Pleistocene sea-level changes affected the Amazon River as far as 2,500 km inland. This results on one hand with the formation of large floodplains of the Amazon and the lower parts of its tributaries during sea-level heights and on the other hand with a deeply incised river system during low sea-level stages. This was most effective since Mid-Pleistocene when the changes of sea-level got stronger. This could be shown from the deeply incised valleys of Negro and Tapajos Rivers. During Last Glacial Maximum the slope of the Amazon below its junction with Tapajos River increased by the factor 10, resulting probably in a braided River. Paleofactors of sediment cores taken from Central Amazonia lakes and from Tapajos River give no hint for a significant change in climate.