Hydrological changes in the Pantanal Wetland, Brazil
by
Mario Luis Assine
UNESP - Universidade Estadual Paulista / Departamento de Geologia Aplicada, Rio Claro, Brazil

The Pantanal is a Quaternary sedimentary basin located at the left margin of the Upper Paraguay River, west-central Brazil. During the late Pleistocene the climate was drier and the landscape was characterized by large braided alluvial fans. Eolian processes were active in abandoned lobes, generating deflation hollows bordered by lunette sand dunes. Eolian activities were probably more effective at the glacial maximum, as indicated by sand thermoluminiscence dating. Avulsion is a natural sedimentary process in the evolution of the alluvial fans and has been occurring since the Pleistocene. The hydrological changes can be interpreted by using morphologic criteria because the original geometry of their channels is preserved as relict forms. The landscape has been changing in the Pantanal area since the end of the Pleistocene, adapting to a more humid and warmer environment prevailing during Holocene. The alluvial fans has remained active depositional systems during the Holocene, although they changed to a low-sinuosity / meandering alluvial fan type. The Pantanal became a complex tropical wetland, composed by meandering fluvial plains, several alluvial fans and small lacustrine systems. Due to water table raising the deflation hollows turned into ponds, and it is probable that isolation from surface waters has caused the salinization of some of them. Many geomorphic features recognizable in satellite and radar images clearly show that river shifting has occurred many times before, especially in the Taquari megafan that cover approximately 50,000 km2 or 37% of the Pantanal area. The Pantanal landscape has been changing as consequence of hydrological changes forced by climatic factors. However, tectonics has also been playing an important role in the development of the Pantanal landscape as a whole, modifying base levels and topographic gradients, and constraining the Paraguay trunk river and the lacustrine systems in the western border of the basin.

Date received: December 16, 2004