Oxbow lake
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An oxbow lake is a U-shaped lake water body formed when a wide meander from the mainstem of a river is cut off to create a lake. This landform is called an oxbow lake for the distinctive curved shape that results from this process. In Australia, an oxbow lake is called a billabong.
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[edit] Formation
When a river reaches a low-lying plain in its final course to the sea or a lake, it meanders widely. Deposition occurs on the convex bank (known as a point bar). This deposition of silt, sand and gravel on the convex bank is the result of a secondary flow across the bottom of the river, sweeping loose material in the direction of the convex bank.
It is instructive to demonstrate this secondary flow using a circular bowl. Partly fill the bowl with water and sprinkle dense particles such as sand or rice into the bowl. Set the water into circular motion with one hand or a spoon. The dense particles will quickly be swept into a neat pile in the center of the bowl. When a fluid follows a curved path, such as around a circular bowl, around a bend in a river or in a tropical cyclone, the flow is described as vortex flow – the fastest speed occurs where the radius is smallest, and the slowest speed occurs where the radius is greatest. Bernoulli's principle explains that the highest pressure occurs where the speed is slowest, and the lowest pressure occurs where the speed is fastest. In vortex flow the highest pressure is where the radius is greatest and the lowest pressure is where the radius is smallest. There is a pressure gradient toward the center of the flow which provides the centripetal force necessary for each parcel of water to follow its curved path. The boundary layer responds to this pressure gradient and as it flows across the streamlines at the bottom of the river or bowl it sweeps loose material toward the convex bank in the case of a river, or toward the center of a bowl of water. This flow of the boundary layer across the floor of the river bed is significantly different to the speed and direction of the primary flow of the river, and is part of the river's secondary flow.
In contrast, both lateral erosion and undercutting occur on the concave bank (or cut bank). Continuous erosion of a concave bank and deposition on the convex bank of a meandering river cause the formation of a very pronounced meander with two concave banks getting closer. The narrow neck of land between the two neighbouring concave banks is finally cut through, either by lateral erosion of the two concave banks or by the strong currents of a flood. When this happens, a new straighter river channel is created and an abandoned meander loop, called a cut-off, is formed. When deposition finally seals off the cut-off from the river channel, an oxbow lake is formed. This process can occur over a time scale from a few years to several decades and may sometimes become essentially static.
The bulges in the border reflect changes in the course of the river; when the river shifted its course and cut off the former channel, the border did not change.
By itself, the word oxbow can also mean a U-shaped bend in a river or stream, whether or not it is cut off from the mainstream.[1][2]
[edit] Examples
The Reelfoot Lake in west Tennessee is an oxbow lake formed when the Mississippi River changed course following the New Madrid Earthquake of 1811–1812. There are many oxbow lakes alongside the Mississippi River and its tributaries. The largest oxbow lake in North America, Lake Chicot (located near Lake Village, Arkansas), was originally part of the Mississippi River.
The town of Horseshoe Lake, Arkansas is named after the horseshoe shaped oxbow lake at the eastern tip of which the town is located.
Cuckmere Haven in Sussex, England contains a widely meandering river with many oxbow lakes, often referred to in physical geography textbooks.
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lv:Vecupe lt:Senvagė nl:Hoefijzermeer ja:三日月湖 pl:Starorzecze ru:Старица (русло) simple:Oxbow lake fi:Juolua sv:Korvsjö
