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 analyzed 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 the image pair above. The top image was acquired on June 11, 1985, by the Thematic Mapper (TM) on the Landsat 5 satellite. The bottom image was acquired on July 13, 2014, by the Operational Land Imager (OLI) on Landsat 8. Turn on the image comparison tool to see how the river course changed over 28 years.
The river flows toward the north (left in these images) 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-colored, 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.
"Natural habitats that exist within floodplains depend on river migration to both renew habitat and maintain the natural functioning of existing habitat," Constantine said. "If we want to ensure a naturally functioning river environment, then we need to ensure the presence of an erodible river corridor and the supplies of riverbed sediment that are required for sustaining meandering river dynamics."
NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey. Caption by Kathryn Hansen.
The largest river on the planet, the Amazon, forms from the confluence of the Solimões (the upper Amazon River) and the Negro at the Brazilian city of Manaus in central Amazonas. At the river confluence, the muddy, tan-colored waters of the Solimões meet the “black” water of the Negro River. The unique mixing zone where the waters meet extends downstream through the rainforest for hundreds of kilometers, and is a famous attraction for tourists from all over the world. The tourism contributes to substantial growth in the city of Manaus. Twenty years ago the large park near the city center (center) lay on the eastern outskirts of Manaus.