Meandering in the Amazon
Over periods of years and decades, the courses of some rivers can be all over the map-literally. These shape-shifting, meandering rivers are naturally dynamic, "working their way across their valley floors, recycling floodplain sediment, and building both river and floodplain habitats as a result," said José Constantine of Cardiff University.
But what causes rivers to meander, and why do some meander more than others? These questions have been the subject of research for more than a century, and several hypotheses and studies have focused on the role of sediments. The Amazon Basin-free of engineering controls and containing a wide range of sediment loads-provides a natural laboratory in which to investigate the relationship.
Constantine and colleagues recently analysed nearly three decades of Landsat imagery of the Amazon Basin. They found that the greater the amount of sediment from external sources (glacial, volcanic, or human activity), the more likely the river was to meander; rivers and streams with lower sediment loads wandered less. Those high-sediment rivers also saw more cutoff events, where crescent-shaped oxbow lakes are formed.
One example is the Rio Mamoré, shown in this image which was acquired on 13 July 2014 by the Operational Land Imager (OLI) on Landsat 8.
The river flows toward the north and receives a large amount of sediment at the confluence with the Rio Grande. The extra sediment enhances the growth of point bars-the lighter-coloured, vegetation-free areas along the inside bends of the riverbank. According to Constantine, these features cause erosion and altered river flow that lead to a 1.7-fold increase in the rate of river migration downstream. In addition, meander cutoff rates doubled.
View the full resolution image, and comparison with Landsat 5.