In the Amazon basin, some rivers run wild. With no dams or levees to control them, they are free to meander—some more than others.
For example, the Rio Mamoré, which flows north across Bolivia, is one such wanderer. It flows from the Andes Mountains and across the Bolivian lowlands into Brazil. Watching this river meander in Landsat images over the past few decades shows us how much a river can meander under natural conditions. The Mamoré has a large sediment load, so it meanders more than most.
These meandering river dynamics are important for maintaining a healthy habitat. The floodplains here depend on the river migration to maintain the wetland habitats.
The growing city of Trinidad, with a population of over 100,000, can be seen in the upper right of these images, just east of the river.
The Mamoré River carries sediment from the rapidly eroding Bolivian Andes. The steep terrain coupled with high river discharge in the Andes causes a high degree of soil erosion.
The high sediment load encourages the growth of point bars, which are seen in these Landsat images as the lighter colored areas along the inside bends of the river. These sandy areas are mostly vegetation-free. Point bars increase the erosion on the opposite side of the river, causing it to further push the river’s course off to the side.
These close-up images show a location on the Mamoré River where several point bars have formed. At the top of these images, this action created a cutoff. Point bars tend to increase the rate of cutoff formation. And the bends that have more sediment (pink) move more than the bends that have less.
View this animation to really see this process in action.
The more a river meanders, the more cutoffs form. Cutoffs form more frequently on rivers that have more sediment. Flanking the Mamoré River in these images are numerous oxbow lakes, formed from these cutoffs.
An oxbow lake starts as a meander, or curve, in the river. Sediment builds up on one side of the curve, called deposition. The river becomes more curvy until the river ultimately loops back onto itself. The river then flows along the straighter path and forms a cutoff. Once the river completes this shortcut, the curve becomes a separate body of water, called an oxbow lake!
Over time, the oxbow lake fills with sediment and detritus and eventually becomes a swamp or bog for a while and then often dries up as the water evaporates.