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Twists and Turns of a River
If you have ever looked down on a natural river valley, you can’t help but notice that the path of the river is anything but straight. (When man interferes with the course of nature, even a river can end up as straight as an arrow.) The serpentine course of the river is made up of a series of twists and turns called meanders. Let's take a look at how and why they form.
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Rivers and the Path of Least Resistance
Rivers are freshwater systems which drain their catchment area through tributary streams. In a rather circular definition, a river’s catchment area refers to the region surrounding the river in which any rainfall run-off will feed into the river in question. Ultimately, rivers carry their freshwater to the seas, feeding into the ocean via estuaries where fresh and salt water mix.
Some rivers have their headwaters (or source) high up in the mountains where they are fed by melting snow or glaciers, if the mountain is high enough to have permanent ice. Due to the force of gravity, all rivers flow from high ground to lower ground and ultimately the sea. Water will not flow up-hill (unless it is pumped) and the water course will always follow the lowest lay of the land, unless man has interceded and is forcing it to flow through a man-made channel. In short, left to its own devices, water always takes the path of least resistance to its flow.
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Rivers don’t just convey water to the oceans. They are also transportation systems for the sediment that enters them. Sedimentation transport rates are highly dependent upon the sediment particle size in question with small, fine sediments being the most easily transported. Sand, gravel and small rocks are transported towards the sea, but their transport rate is much slower since more force (such as that found in spring flooding, or a strong current) is needed to move them.
The earth that a river traverses is anything but smooth and flat and there are structures such as boulders, which cause the path of a river to change as the water finds the path of least resistance. An idealized river flows with uniform velocity (laminar flow); consequently, it is not easy to imagine why it should start to deviate and go on to form meanders. However, laminar flow is more of a hydrodynamic theorem than a practical view of real world systems. Indeed, even Coriolis forces (due to the Earth’s rotation) will cause one side of the river to flow marginally faster than the other. When a river negotiates an obstacle, such as a large boulder, the relative speed of the water flow at the edges of the water body will change. The water flowing closest to (and around) the obstacle will be flowing more slowly than water on the opposite side of the river. It will have less momentum than water on the other side of the river (and therefore, less energy).
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Construction and Destruction
The faster flowing water on the opposite side to the obstacle has higher energy and is more effective at eroding the river bank and removing any silt, which may have been deposited there. The bank will become increasingly concave as this process continues. Material that has been eroded from the bank will be carried downstream and eventually deposited in slack water, where the flow rate is lowest. Gradually, a bank will start to build where the material is deposited, forming a bar. The displaced material tends to be deposited along the sides of the river and at the convex curve of the next bend.
The nature of the terrain and soil type that the river is passing through will clearly have an influence on the rate at which a bank will erode. Light, finely divided soils are likely to erode quickly, but they can be stabilized by vegetation (particularly root systems) at the water’s edge. A fast-flowing, highly energetic river is likely to erode its banks more rapidly than a placid, slow-flowing one.
In conclusion, the formation of a meander is a process of both erosion and deposition. Of course, the process is dynamic since the erosion and depositional processes change the nature of the river flow which caused them in the first place. The process of erosion and deposition can lead to the formation of an oxbow lake (a stagnant pond), where the river breaks through at two adjacent bends to adopt a line of lower resistance (this is illustrated in reference 2).
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- The Sierra Club: http://www.sierrapotomac.org/W_Needham/MeanderingRivers.htm
- Graphic showing meander formation: http://www.cleo.net.uk/consultants_resources/_files/meander4.swf