MEANDERS Alice & Emily

An aerial view of the beautiful river meanders in the Amazon basin:




Definition:
Meanders are sinuous bends in a river with a faster flow of water on the outside channel leaving the inside to be slower therefore depositing sediments.  They are usually in the middle and lower course where there is more lateral erosion. Lateral erosion by meanders leads to the widening of the valley floor i.e flood plains. They become wider and larger over time.
Formations of Meanders:

·      The water flows faster on the outside bend of a river because it has further to travel, its speed and erosive capacity is therefore greater.

·      This erodes the outside of the meander, producing a river cliff and deeper channel on that side.

·      On the inside bend, the water is slower and shallower, allowing the river to deposit some of its load, the deposits form what is known as a slip-off slope.

·      Water also flows in a horizontal corkscrew motion called helicoidal flow, in certain sections of a meandering river.

·      This transports more eroded material, from the river cliff of one meander, to the slip-off slope of the next.

·      This process makes the meanders bigger and assists in the slow migration downstream.

Animation of a meander: http://www.wiley.com/college/strahler/0471480533/animations/ch17_animations/animation1.html

But why do rivers meander?

·      One theory is that they are created when the main current in a river swings from side to side to avoid riffles.

·      Riffles are the shallower section of rivers where rocks often break the surface and slow down the flow of the water.

On the outside of the curve the velocity, and therefore the erosion, of the current is greatest. Here the river cuts into the outside bank, producing a river cliff.

On the inside of the curve the current is slow and so it deposits any transported material, building up a gentle slip-off slope.


Formation of meanders in pictures:


Examples of Meanders:  

The Napa River, near the Amazon.

Facts:
  • 20 to 25 percent of water that runs off the Earth's surface comes from the Amazon
  • It  has 1,000 tributaires and the river's total drainage basin is nearly twice as large as the area drained by any other river in the world
  • The annual average discharge of 180,000 cubic m per second at its 240 km wide mouth
A meander on a smaller scale the River Dart, Dartmoor. 

   


FUN FACT: 
The word meander comes from the name of the River Menderes in Turkey because of its winding course.

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