Tuesday, 25 September 2012

Meanders and Oxbow Lakes

A meander is a bend in a river. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. 

The eroding sediments from the outside of a bend are deposited on the inside. The result is a snaking pattern as the stream meanders back and forth. 

When a meander gets cut off from the main stream, an oxbow lake is formed.

The stages of formation of an oxbow lake:

Stage 1 - Continual erosion of the outer banks of a meander by processes such as hydraulic action causes the neck of the meander to become narrower and narrower.

Stage 2 - Eventually as the neck becomes increasingly narrow, the two outer bends meet and the river cuts through the neck of the meander. The water now takes the shortest route rather than flowing round the bend. The fastest current is in the centre and deposition is likely at the sides. This deposition begins to seal off the old meander from the rivers new channel.

Stage 3 - Eventually deposition completely seals off the old meander and the old meander bend is left isolated as an ox-bow lake. Over time this feature is likely to fill with sediment and dry up (except for periods of heavy rain). The feature left behind when the water dries up is known as a meander scar.