An S-shaped pattern formed when a stream flows through channels is called

meander

Explanation:

A meander is a series of regular sinuous curves in the channel of a stream/river/watercourse, thereby forming a rounded or "S"-shaped bend curve. Meanders are common in alluvial areas that have low gradients, where they are formed by the dynamics of the watercourse. The meander belt is the zone within which a meandering watercourse occupies on its floodplain. The sinuosity is a measure to calculate the degree of meandering in the channel of a watercourse.

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