Meander

The man with the astrolabe told me
That Roger was coming to sing
And Gwynneth would dance like an angel
And all the nine tailors would ring

He said the queen valued my message
And was graciously pleased to observe
That I combined a quite fitting description
With felicitous subject and verb

And the dolphins were swimming in brandy
And the tigers were roaring in tune
And the elephant's progress was stately
As he left not a mark on the dune

The woman with forty-three children
Was feeding them porridge and tea
She said that she'd give them some shortbread
If only they'd share it with me

The sun was unceasingly scorching
The snow was all melted away
The ants were ferocious and biting
While the mantis was waiting to pray

My mother said I was a darling
My father said I was a fool
My teacher just shook his head sadly
And drew a straight line with his rule

I haunted my books for an answer
I sought out the words of the wise
But they whirled on the page like a dervish
Obscuring the truth from my eyes

I walked to the top of a mountain
The man with the astrolabe grinned
The clouds scooted on the horizon
Gone with the breath of the wind

The classical Greek name for a river in southwestern Anatolia (now Büyük Menderes¹ in Turkey), at whose mouth the ancient city of Miletus grew. This river winds lazily through its valley (produced by a graben) and its name has become the generic term in geomorphology for the winding of a river within its floodplain.

A meandering channel is a form that people will instantly associate the word "river" with, since nearly all mature rivers with floodplains meander in some fashion. The only necessary condition is a floodplain, and a high-enough ratio of water plus suspended load to bed load. However, high bed load streams frequently take on a braided channel, which can be viewed as the chaotic interaction of many meandering channels. As bed load conditions change, a meandering channel can change into a braided one, and a braided channel can simplify itself into a meandering one.

Morpohology

 
                                                                 .-----.__
               _.-----.__                       _.-----.__      / .----._ \
_             / .-----._ \                     / .-----._ \    / /  B   ( (
 \           / /  B     ) )    _.-----._      /R/   B    \ \  ( (        \ \     Downstream
\ \         ( (        / /   .'_.-----._`.   / /         | |   \ \        \ \
 | |    B    \R\      / /   / /         | | ( (          |R|    \ \        ) )
 \ \          ) )    /R/   ( (          | |  \ \         / /     \R\      /R/
  `.`-.____.-' /    / /     \R\         `.`.__) \       / /       \ \    / /          _.-----
    `-.____.--'    ( (       \ \          `-.___/      ( (         \ \  ( (          / .-----
                    \ \       \ \      _       _|       \ \         ) )  \ \         | |
                     \ \       \ \    //       \\        \ \   B    / /   \ \        | |
Upstream              \ \   B   ) )  ((         \`.       `.`-.___.' /     \ \   B   | |
                       `.`-.___/ /    \`.        ) )        `-.____.'       `.`-.___.' /
                         `-.___.'      \ \      / /                           `-.___.-'
                                        `.`-.__/ / OXBOW
                                          `-.__.'    LAKE

On the insides of the meanders (labeled B in the diagram) you will invariably find sand and gravel bars. (explained below). The river makes most of its vertical descent in the stretches between meanders called "riffles" (labeled R in the diagram). An Oxbow Lake or bilabong is a portion of a meander that has been cut off from the river.

The root causes of meandering

No single factor causes a river to meander. All of the following play a part:

Hydraulic drag

may be the only necessary factor. Friction between the river water and the underlying surface makes water slow down and move sideways, like a truck inching down a switchbacked mountain road. Experiments with running pure water down an inclined sheet of glass reveals that streams will meander with no other assistance.
 

Irregularities in the antecedent drainage.

Theoretical rivers that form from small rills are forced to run around obstacles, and are channeled by cracks in the ground. There is enough chaotic input into the formation of any stream to cause it to meander. As the young drainage system develops, stream piracy causes a river's principal flow path to change, as an aggressively extending tributary captures a larger but higher stream.

But the "meanders" caused by initial conditions cause the valley to meander with the river. As the valley fills in with sediment to form a floodplain, these valley meanders tend to straighten out.

Erosion and Deposition

Water on the outside of a bend in the river invariably moves faster than the water on the inside of the bend. A greater speed causes a greater ability to erode the bank on that side. It also increases the ability of the water to transport sediment. Furthermore, river water soaks into the alluvium of the river banks on the outside of the meander, weakening the outside bank further, especially during a flood.

While the river bank on the outside of the bend tends to get eaten away, coarser-grained particles tend to get deposited near the bank on the inside of the bend (especially as a flood recedes), forming point bars (labeled B in the diagram). On the inside of the bends, lower speed reduces the ability to transport sediment. As a result, the outsides of meanders are usually vertical bluffs under active erosion, while the tips of the bars are just above normal river level. Eventually, the bars are covered with silt during a flood, grow vegetation, and support vegetation, becoming another part of the floodplain.

Processes that affect and reinforce meandering

Meanders

Why do the "meanders" of youthful river valley contribute to the formation of meanders in the floodplain? Because meanders cause meanders: Meandering is a self-reinforcing process. Forces in the river channel amplify any small deviation in the river channel into broad, sweeping loop. Eventually, two meanders will intersect each other, cutting off a loop of the river in an oxbow lake.

Flooding

Floodplains form because the river frequently overflows its banks, depositing fine sediment in the area beyond its banks. In a way, what we would call the "river" is merely the thalweg of the floodplain. Meanders always develop faster during a flood; oxbow cuttofs usually happen during such events.

Downstream migration

The ability of a river to erode the outward bank of a meander is slightly greater on its downstream side. Because of this, the meanders tend to "droop" and migrate downstream.

Strange meanders

The processes described above cannot explain all meanders. Some meanders take forms so strange that other factors must be involved.

Meanders affected by geologic structure

The underlying bedrock can control the form of some river meanders. For example, meanders of the Shenandoah River and other rivers in the Appalachian Ridge and Valley Province have a peculiar, "squared-off" look. They do not droop downstream. Although no one is sure why yet, the best explanation seems to be that parallel sets of rock joints in the subsurface geology keep the meanders in line.

Meanders affected by tides

As sea level rises since the last glaciation has flooded many coastal river valleys, creating estuaries and coastal swamps, tides cause water to flow through the former stream channels in boh directions. And so, the "downstream" erosive force that causes the meanders to "droop" on a floodplain becomes an upstream force twice a day. This results in channels with rounded, craed meanders with crazily-meandering tributaries. Examples of tidally-affected meanders appear all along the coastline of the United States's Atlantic Coastal Plain, such as Delaware's Bombay Hook National Wildlife Refuge, and Maryland's Blackwater National Wildlife Refuge.

Multi-phase meanders

A meandering river that finds its flow suddenly reduced will sometimes begin meandering within the meanders, forming "two-phase meanders":

                               .-.   .-.   
___        _.-._   _          (( \\_// \\      
--.\      /_/ \_`-'_`._.-._    )) `-'   ))     \\
  ((     ((     `-' \_.-._ \  ((       //       ))
   ))     ))              ))   ))     ((       ((
  ((     ((              ((   ((       )) .-.   \\
   \\_.-._))              ))   ))     // // \\   ))
    \_.-._.'             ((   ((     (( ((   \\_//
                          \\___))     \\_))   `-'
                           `-.-'       `-'

Incised meanders

When a mature river valley undergoes sudden tectonic uplift, the river is forced to suddenly cut down into its valley. Since all of the river's energy is spent on cutting down, less energy is spent on moving the meanders. The result is a rejeuvenated stream valley with a sinuous character, just like meanders in a floodplain. The "Goosenecks" of the San Juan River in Southwestern Colorado are an extreme example of incised meanders. On a map, the river channel appears to meander like any other mature river. Howver, inside the meanders are 1,000 foot high masses of rock!

¹Büyük is Turkish for big; Küçük (little) Menderes is the Turkish name for the fabled Scamander river, which flows past the site of Troy.

Me*an"der (?), n. [L. Maeander, orig., a river in Phrygia, proverbial for its many windings, Gr. : cf. F. m'eandre.]

1.

A winding, crooked, or involved course; as, the meanders of the veins and arteries.

Sir M. Hale.

While lingering rivers in meanders glide. Sir R. Blackmore.

2.

A tortuous or intricate movement.

3. Arch.

Fretwork. See Fret.

 

© Webster 1913.

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Me*an"der, v. t.

To wind, turn, or twist; to make flexuous.

Dryton.

 

© Webster 1913.

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Me*an"der, v. i. [imp. & p. p. Meandered (?); p. pr. & vb. n. Meandering.]

To wind or turn in a course or passage; to be intricate.

Five miles meandering with a mazy motion Through wood and dale the sacred river ran. Coleridge.

 

© Webster 1913.