Rivers 2

Meanders and Ox Bow Lakes

A meander is formed when the moving water in a river erodes the outer banks and widens its valley creating a meander. A stream of any volume may assume a meandering course, alternatively eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an ox bow lakeis formed. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

Formation:

Meander formation is a somewhat equivocal term referring to the natural factors and processes that result in meanders. The waveform configuration of a stream is constantly changing. Once a sinusoidal channel exists it undergoes a process during which the amplitude and concavity of the loops increase dramatically due to the effect of helicoidal flow in increasing the amount of erosion occurring on the outside of a bend. In the words of Elizabeth A. Wood
... this process of making meanders seems to be a self-intensifying process ... in which greater curvature results in more erosion of the bank, which results in greater curvature ...
The helical flow is explained as a transfer of momentum from the inside of the bend to the outside. As soon as the flow enters the bend some of its momentum becomes angular, the conservation of which would require an increase of velocity on the inside and a decrease on the outside, exactly the opposite of what happens. Instead centrifugal force superelevates the surface on the outside, moving surface water transversely into it. This water moves down to replace the subsurface water pushed back at the end of the bend. The result is the scouring helical flow, and the greater the curvature, the greater the angular momentum and the stronger the cross-current.
The question of formation is why streams of any size become sinuous in the first place. There are a number theories, not necessarily mutually exclusive.

Oxbow lakes
Oxbow lakes are created when growing meanders intersect each other and cut off a meander loop, leaving it without an active cutting stream. Over time, these oxbow lakes tend to dry out or fill in with sediment.