Meander

What is a meander?

Meanders are sinuously curves in which water streams move among, transporting high quantities of sediments in suspension (1). These streams flow through soft and deep stream beds, carrying mud (lime and clay), sand and, sometimes, gravel. 

A meander is a distinctive, undulating pathway of a waterway
Figure 1. Aerial photograph of a meander.

How is it formed and how does it work?

Water streams erode the land in three ways: by abrasion, pulling and corrosion. Given this type of erosional processes, it comes the transportation of sediments, which depends of the size of the particles. In large sized sediment particles, it develops as rolling and sliding; medium sized sediment particles travel through saltation; and small sized sediments particles are transported as a dissolved load or by suspension (2).

Figure 2. Transportation of sediments through a water stream according to the size of particles.

The ability of a stream to transport particles can be described in two ways:

  • By its capacity: which refers to the maximum amount of sediments that may be carried. As larger the water that flows in the stream is, the higher its capacity to transport the sediment. Large rivers with high flow speeds have a major capacity.
  • By its competence: which measures the ability of a stream to transport particles according to the size of these, rather than the quantity which is carried away. The key lays in the water speed; fast water streams have a larger competence that the slower ones, despite the size of the water stream beds. The competence of a stream increases proportionally to the mathematical square of its speed (3).
Figure 3. Diagram that shows the relation between the river speed (Y axis), the particle size (X axis) and the total amount of particles eroded, transported and deposited.

As meandered streams are mostly found in plains, the speed of the flow is not big enough so as to carry big sized particles, which explains why landforms found in this kind of water streams are composed by thin particles. 

This makes it easier for the river to evolve in its form within time and space as the horseshoes migrate through the erosional plain in a horizontal way rather than a vertical way, as shown below (4):

Figure 4. Evolution of a meander.

The side of the stream bed which is eroded with more energy is the external side, where the speed and the turbulence are higher (5). The external side is mostly eroded, especially during the growth season. 

Figure 5. Water stream speed and form.

What landforms are created by this process?

  • As long as this margin is undercut, it acquires an inclination which will, afterwards, lead into a landslide into the stream. Given that the external side of the margin is actively eroded, it is called cut bank (6) (7).
Figure 6. Schema of a floodplain with the basic landforms.

Cut banks are found along meandering streams. They are located on the outside of a stream bend, opposite the slip-off slope on the inside of the bend. Their shape is similar to a small cliff, and are formed by the erosion of soil as the stream collides with the river bank. As opposed to a point bar, which is an area of deposition, a cut bank is an area of erosion

Figure 7. Satellite image showing a meander with a cutbank on the external margin and a point bar in the internal margin of the water stream.

Normally, cut banks are nearly vertical and often expose the roots of nearby plant life. Often, particularly during periods of high rainfall and higher-than average water levels, trees and poorly placed buildings can fall into the stream due to mass wasting events. Given enough time, the combination of erosion along cut banks and deposition along point bars can lead to the formation of an oxbow lake.

Not only are cut banks steep and unstable, they are also the area of a stream where the water is flowing the fastest at a higher pressure and often deeper, making them rather dangerous. Material eroded here is deposited downstream in point bars

  • point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than, towheads, or river islands.
Figure 8. Aerial photograph showing a point bar.

Point bars are composed of sediment that is well sorted and typically reflects the overall capacity of charge transportation of the stream. They also have a very gentle slope and an elevation very close to water level. Since they are low-lying, they are often overtaken by floods and can accumulate sediments during times of high water levels. Due to their gentle slope and the fact that the water speed is slow in the shallows of the point bar they are popular rest stops for boaters and rafters. 

Point bars are formed as the secondary flow of the stream sweeps and rolls sand, gravel and small stones laterally across the floor of the stream and up the shallow sloping floor of the point bar.

  • An oxbow lake is a U-shaped lake that forms when a wide meander of a river is cut off, creating a free-standing body of water (9). This landform is so named for its distinctive curved shape, which resembles the bow pin of an oxbow.
Figure 9. Evolution of a meander and an oxbow lake.

The word “oxbow” can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream. 

  • When several processes of erosion, transport, deposition and migration of meanders occur, they are formed meander scars (10). They are formed by the remnants of a meandering water channel. They are characterized by “a crescentic cut in a bluff or valley wall, produced by a meandering stream. They are often formed during the creation of oxbow lakes.
Figure 10. Floodplain and its erosional and depositional landforms.
Figure 11. Amazon River meandered water streams.

Normally, we would expect a water stream with a high sinuosity to be found in a floodplain and a wide valley, but some rivers exhibit meandered stream beds which flow in narrow and high sloped valleys. How are there shapes formed? Originally, it is probable that the meanders were developed in the floodplain which was relatively close to the base level (the lower limit of an erosional process). A modification in the base level made the stream to erode in a vertical way (over a horizontal way). It may be developed in two ways:

  • Or the base level descended
  • Or the land in which the stream flows uplifted. This last cause leads to incised meanders (12).
Figure 12. Aerial photograph of and incised meander.

Conclusion

Meanders are formed mainly in plains by erosion, transport and deposition of small and medium sized particles, such as sand, lime and clay. The water stream erodes the external margin of the stream bed due to the higher speed in which it flows, transports the sediment and afterwards deposit it in the internal margin (where the speed of the flow is slower). This leads to the creation of cut offs and meander scars in the external margin; point bars in the internal margin; and oxbow lakes (when two parts of the stream come closely together).

Read more about the Energy of a River

Frequently Asked Questions

What is a meander in geography?

A meander is a winding curve or bend in a river’s course, often characterized by a sinuous, snake-like shape.

How do meanders form?

Meanders form through the erosional and depositional processes of a river, as it erodes the outer banks of a bend and deposits sediment on the inner banks.

What role do meanders play in river ecosystems?

Meanders create diverse habitats and enhance ecological diversity by providing areas for sediment deposition, fostering the growth of riparian vegetation, and supporting aquatic life.

Can meanders change over time, and if so, how?

Yes, meanders can change due to factors like erosion, sediment deposition, and human interference. These changes may result in meander cutoffs or shifts in the river’s course.

Are meanders unique to Earth, or are they found on other planets?

Meanders are not unique to Earth; similar features have been observed on Mars and other planetary bodies with liquid flows in the past.

Bibliography and image sources

Cite/Link to This Article

  • "Meander". Geography Revision. Accessed on March 28, 2024. https://geography-revision.co.uk/a-level/physical/meander/.

  • "Meander". Geography Revision, https://geography-revision.co.uk/a-level/physical/meander/. Accessed 28 March, 2024.

  • Meander. Geography Revision. Retrieved from https://geography-revision.co.uk/a-level/physical/meander/.