Meanders are sinuous bends in a river's middle and lower courses.
In low flow conditions, alternating pools and riffles are formed along the river bed. The river channel is deeper in pools so it has greater energy and more erosive power. Energy is lost as the river flows over a riffle because of friction. These cause the river's flow to become uneven and maximum flow to be concentrated on one side of the river, causing lateral erosion on one side, creating an outer concave bank. Deposition takes place on the other side of the bend, creating a convex bank.
The cross-section of a meander is asymmetrical. The outer bank forms a river cliff or a bluff with a deep pool close to the bank, mainly because of the fast flow, hydraulic action and abrasion. The inner bank is a gently sloping deposit of sand and gravel, called a point bar.
Meanders are maintained by a surface flow of water across to the concave outer bank with a balancing subsurface return flow back to the convex inner bank. This corkscrew-like movement of water is called helicoidal flow. In this way, eroded material from the outer bank is transported away and deposited on the inner bank.
The combination of erosion and deposition exaggerates the bends until large meanders are formed. Sometimes, oxbow lakes are formed when the neck of the loop of a meander is broken through, and the new cut eventually becomes the main channel, leaving the formed channel sealed off by deposition.
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