Sediment supply drives floodplain evolution in Amazon Basin

A new study of the Amazon River basin shows lowland rivers that carry large volumes of sediment meander more across floodplains and create more oxbow lakes than rivers that carry less sediment.

The findings have implication for the Amazonian river system, which may be significantly altered by proposed mega-dams that would disrupt sediment supplies.

Researchers from Cardiff University’s School of Earth and Ocean Sciences examined 20 reaches within the Amazon Basin from Landsat imagery spanning nearly 20 years (1985 to 2013).

They found rivers transporting larger amounts of sediment migrated more, and noted that channel movement did not depend on either the slope of the channel or the river discharge.

The research gives scientists insight into the contrasting behavioural properties of rivers where sediment is an imposed variable – e.g. resulting from glacial, volcanic, or human activity – and rivers were the main sediment supply is from local bank erosion.

Dr José Constantine, Lecturer in Earth Sciences at Cardiff University’s School of Earth & Ocean Sciences and lead author of the paper said: “We found that the speed at which the meanders migrated for each of the rivers studied depended on the river’s supply of sand and silt. The meanders of rivers carrying more sediment migrated faster than those carrying less sediment, and were also more frequently cut off and abandoned to form U-shaped lakes. If sediment loads are reduced — by a dam, for example — meander migration is expected to slow, and thus the reshaping of the floodplain environment is affected.

Scientists discover Amazon river is 11 million years old

Researchers at the University of Liverpool have discovered that the Amazon river, and its transcontinental drainage, is around 11 million years old and took its present shape about 2.4 million years ago.

University of Liverpool researchers, in collaboration with the University of Amsterdam and Petrobras, the national oil company of Brazil, analysed sedimentary material taken from two boreholes near the mouth of the river to calculate the age of the Amazon river and the Amazon deep sea fan.

Prior to this study the exact age of the Amazon, one of the two largest rivers in the world, was not known. Until recently the Amazon Fan, a submarine sediment column around 10km thick, had proven difficult to penetrate. New exploration efforts by Petrobas, however, have lea to two new boreholes being drilled near the mouth of the Amazon – one 2.5miles (4.5km) below sea level – which resulted in new sedimentological and paleontological analysis of samples from the river sediment.

“River sediment records provide a unique insight into the palaeoclimate and geography of the hinterland,” said Jorge Figueiredo from the University’s Department of Earth and Ocean Sciences

“This new research has large implications for our understanding of South American paleogeography and the evolution of aquatic organisms in Amazonia and on the Atlantic coast. The origin of the Amazon river is a defining moment: a new ecosystem came into being at the same time as the uplifting Andes formed a geographic divide.”

The study was published in the scientific journal, Geology, in July 2009.