The town of Shrewsbury
Example of a Meander:
Shrewsbury is a county town lying on the river Severn in the West Midlands region and is home to around 71 000 inhabitants. It is placed inside a meander, and was originally a medieval market town. The meander was very helpful for imports and exports as well as travel in the medieval ages.
Since 1990, Shrewsbury has experienced severe flooding problems becuse of its location - in 2005 the defense systems were increased dramatically to hold back the potential flooding. The land is very fertile due to all the layers of sediment and silt that have been forming over many years. Therefore, farming is a possibility on flood plains, around meanders. These crops must be resilient against high discharge in times of flooding.
A typical exam question:
With the help of well-labelled diagrams, describe and explain the formation of meanders.
Definition of meanders:
They are curves or bends along a river which are usually found in the middle or lower course of a river.
Formation of meanders - pools and riffles
Meanders originate from fairly straight channels with developing pools and riffles. Water never flows in a straight line even in an apparently straight river channel. It twists and turns around bedload such as cobbles and boulders. This results in areas within a river where there are differences in velocity of river flow, especially in rivers that are carrying coarsier river load.
In deeper parts of the river known as pools, fine sediments such as silt are deposited. In shallower parts of the river known as riffles, the river flows over coarser river load such as gravel, rocks and pebbles.
The coarser river load at the riffles slows river velocity down. However, water flow speeds up after it flows past the riffles. Further encouraged by the steeper gradient at the riffles, the higher velocity after the obstruction at the riffles allows more erosion to take place and the area just after a riffle turns into a pool.
The resultant difference in the velocities of water flow then causes variations in erosion and deposition. The water also starts to flow from side to side and the river begins a more winding course.
Formation of a meander - erosion at the concave (outer) bank and deposition at the convex (inner) bank
Water tends to flow faster around the bends, on the outer banks of the river. As water is being flung rounds the bends at higher velocity, they develop greater energy to erode the outer banks. The eroded material slumps down into the river and over time, the outer bank deepens and turns into a river cliff. The river cliff gets a concave shape and this is why the outer bank of a meander is also known as the concave bank.
River Cliff at the concave bank due to erosion by the river
In contrast, at the inner banks of the river, deposition takes place as the water there is slow-moving. Furthermore, helicoidal flow (see next post) brings eroded river load from the outer banks and deposit them at the inner banks, causing the inner banks to become shallower. Water flowing at the shallower inner banks thus encounters more friction, and thus, further encourage more deposition. Over time, the deposits form a gentle slip-off slope (otherwise known as point bar). The inner bank of a meander is thus also known as the convex bank because its shape is created by the deposition of materials on a slope at this part of the meander.
Slip-off slope (or point bar) at the convex bank
Hence, the formation of a steep river cliff on the concave bank and a gentle slip-off slope on the convex bank of a meander cause it to become more asymmetrical over time.
How do meanders change over time? (Advanced Geography, Garrett Nagle)
meanders may migrate and erode river cliffs
meanders may migrate laterally and erode flood plains
after repeated flooding, meanders can become more pronounced and give rise to the formation of a cut off and ox bow lake (see next post).
Cross sections of a meandering river
Meander Patterns:
In fact, there are many meander patterns! The diagram below reveals 8:
Check out the following animations to have a clearer idea of how meanders and ox-bow lakes (see next post) are formed:
- Animation showing how erosion and deposition at the concave and convex banks of a river cause it to be more sinuous (winding and curving) though time. The animation also illustrates how a river not only changes its shape but also shift laterally overtime.
- Another animation focusing on the development of a meander into a cut off and an ox bow lake.
