meander

A meander is a bend in a river, also known as an oxbow loop, or simply an oxbow. A stream or river flowing through a wide valley or flat plain will tend to form a meandering stream course as it alternatively erodes and deposits sediments along its course. The result is a snaking pattern as the stream meanders back and forth across its floodplain. When a meander gets cut off from the main stream body, an oxbow lake is formed.

Formation

The depositional slip off slope is on the left whilst there is a small river cliff to the right

Geographers have been unable to agree on the exact reason that meanders form. However the formation of meanders is linked to the existence of pool-riffle sequences where meanders form and hard rock, when the meander meets the hard rock it then bends into another area. Thus forming a meander. The existence of helicoidal flow in increasing the amount of erosion occurring on the outside of a bend also plays a part in meander formation

One theory of meander development is that the formation of sandbars causes the river to meander.^[1]

River cliffs and slip off slopes

Most meanders occur in the lower course of the river. Erosion is greater on the outside of the bend where velocity is greatest. Deposition of sediment occurs on the inner edge because the river, moving slowly, cannot carry its sediment load, creating a slip-off slope (Point Bars). The faster moving current on the outside bend has more erosive ability and the meander tends to grow in the direction of the outside bend, forming a river cliff. This can be seen in areas where willows grow on the banks of rivers; on the inside of meanders, willows are often far from the bank, whilst on the outside of the bend, the roots of the willows are often exposed and undercut, eventually leading the trees to fall into the river. This demonstrates the river's movement. Slumping usually occurs on the concave sides of the banks resulting in mass movements such as slides.

View of an incised meander in the Meuse in the French Ardennes

Meanders and oxbow lakes in the Songhua River

If the region later undergoes tectonic uplift, the meandering stream will again resume downward erosion. The meandering pattern will remain as a deep valley known as an incised meander. Rivers in the Colorado Plateau and streams in the Ozark Plateau are noted for these incised meanders. Incised meanders can also be formed when global falls in base level due to fall in sea levels.

Sometimes an incised, also known as entrenched, meander is cut off. When it is, the resulting landform is called a rincon. They are created when a river erodes through the narrow neck of land between the ends of a loop, leaving the loop without an active cutting stream. One dramatic rincon on Lake Powell is called "The Rincon."

Origin of term

The term derives from the river known to the ancient Greeks as (Μαίανδρος) Maiandros or Maeander, characterised by a very convoluted path along its lower part of the river. As such, early usage of the term, from the late 1500s, referred to convoluted and windy speech and ideas, as well as the geomorphological feature. The Meander is located in present-day Turkey, south of Izmir, near the ancient Greek town of Miletus. It is known in Turkey as the Menderes River.

Meander ratio

Meanders of the Rio Cauto at Guamo Embarcadero, Cuba.

Meander ratio is a means of quantifying how much a river or stream meanders (how much its course deviates from the shortest possible path). It is calculated as the length of the stream divided by the length of the valley. A perfectly straight river would have a meander ratio of 1 (it would be the same length as its valley), while the higher this ratio is above 1, the more the river meanders.

Sinuosity Index is the measure of meandering of a stream - when the index is between 1 to 1.3 the river is sinuous, but if the sinuosity index is greater than 1.3 then the river is said to be meandering.

See also

• Riffle-pool sequence
• Meander migration
• Helicoidal flow
• Baer's law

References

• Lewis C. Shaw, Pennsylvania Gazetteer of Streams Part II, Commonwealth of Pennsylvania, Department of Environmental Resources, Bulletin No. 16, 1984, page 8.
• Luna B. Leopold & W.B. Langbein, River Meanders, Scientific American, June 1966, page 60

1. ^ http://www.s-cool.co.uk/topic_quicklearn.asp?loc=ql&topic_id=13&quicklearn_id=2&subject_id=6&ebt=&ebn=&ebs=&ebl=&elc=

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