meander n : a curve in a stream v : to move or cause to move in a sinuous, spiral, or circular course; "the river winds through the hills"; "the path meanders through the vineyards"; "sometimes, the gout wanders through the entire body" [syn: weave, wind, thread, wander]

English

Etymology

From Maeander from Μαίανδρος (Maiandros) - a river in Asia Minor (present day Turkey) known for is winding course. (Turkish: Büyük Menderes Nehri)

Pronunciation

• a UK /miˈændə(ɹ)/, mi"

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 the lower reach. As such, even in Classical Greece the name of the river had become a common noun meaning anything convoluted and winding, such as decorative patterns or speech and ideas, as well as the geomorphological feature. Strabo said: "... its course is so exceedingly winding that everything winding is called meandering."

Meander geometry

The technical description of a meandering watercourse is termed meander geometry or meander planform geometry. It is characterized as an irregular waveform. Ideal waveforms, such as a sine wave, are one line thick, but in the case of a stream the width must be taken into consideration. The bankfull width is the distance across the bed at an average cross-section at the full-stream level, typically estimated by the line of lowest vegetation.

Formation

Meander formation is a somewhat equivocal term referring to the natural factors and processes that result in meanders. The waveform configuration of a stream is constantly changing. Once a sinusoidal channel exists it undergoes a process during which the amplitude and concavity of the loops increase dramatically due to the effect of helicoidal flow in increasing the amount of erosion occurring on the outside of a bend. In the words of Elizabeth A. Wood:... this process of making meanders seems to be a self-intensifying process ... in which greater curvature results in more erosion of the bank, which results in greater curvature ...

The question of formation is why streams of any size become sinuous in the first place. There are a number theories, not necessarily mutually exclusive.

Stochastic theory

The stochastic theory can take many forms but one of the most general statements is that of Scheidegger:The meander train is assumed to be the result of the stochastic fluctuations of the direction of flow due to the random presence of direction-changing obstacles in the river path. Given a flat, smooth, tilted artificial surface, rainfall runs off it in sheets, but even in that case adhesion of water to the surface and cohesion of drops produce rivulets at random. Natural surfaces are rough and erodable to different degrees. The result of all the physical factors acting at random is channels that are not straight, which then progressively become sinuous. Even channels that appear to be straight have a sinuous thalweg that leads eventually to a sinuous channel.

Equilibrium theory

In the equilibrium theory, meanders decrease the stream gradient until an equilibrium between the erodability of the terrain and the transport capacity of the stream is reached. A mass of water descending must give up potential energy, which, given the same velocity at the end of the drop as at the beginning, is removed by interaction with the material of the stream bed. The shortest distance; that is, a straight channel, results in the highest energy per unit of length, disrupting the banks more, creating more sediment and aggrading the stream. The presence of meanders allows the stream to adjust the length to an equilibrium energy per unit length in which the stream carries away all the sediment that it produces.

Geomorphic/Morphotectonic theory

Geomorphic refers to the surface structure of the terrain. Morphotectonic means having to do with the deeper, or tectonic (plate) structure of the rock. The features included under these categories are not random and guide streams into non-random paths. They are predictable obstacles that instigate meander formation by deflecting the stream. For example, a sandbar (geomorphic) might deflect the stream, causing or influencing a meander pattern, or the stream might be guided into a fault line (morphotectonic).

Associated landforms

Erosion Mechanics

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.

Deposits

Incised meanders

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.

Rincons

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."

Scroll-bars

Scroll-bars are a result of continuous lateral migration of a meander loop that creates an asymmetrical ridge and swale topography on the inside of the bends. The topography generally parallel to the meander and is related to migrating bar forms and back bar chutes which carve sediment out from the outside of the curve and deposit sediment in the slower flowing water on the inside of the loop, in a process called lateral accretion. Scroll bar sediments are characterized by cross-bedding and a pattern of fining upward . These characteristics are a result of the dynamic river system, where larger grains are transported during high energy flood events and then gradually die down, depositing smaller material with time (Batty 2006). Deposits for meandering rivers are generally homogenous and laterally extensive unlike the more heterogenous braided river deposits There are two distinct patterns of scroll-bar depositions; the eddy accretion scroll bar pattern and the point-bar scroll pattern. When looking down the river valley they can be distinguished because the point-bar scroll patterns are convex and the eddy accretion scroll bar patterns are concave . Scroll bars often look lighter at the tops of the ridges and darker in the swales. This is because the tops can be shaped by wind, either adding fine grains or by keeping the area unvegetated, while the darkness in the swales can be attributed to silts and clays washing in in high water periods. This added sediment in addition to water that catches in the swales is in turn is a favorable environment for vegetation that will also accumulate in the swales. [ Monocline310 (talk)]

Derived quantities

The meander ratio or sinuosity index 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.

Notes

References

• Luna B. Leopold & W.B. Langbein, River Meanders, Scientific American, June 1966, page 60

See also

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

External links