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As downcutting streams begin to approach base level, the erosional action begins to act more laterally than vertically. The stream begins to meander with respect to the straight stream course. Meanders often begin as a result of existing topographic control, but as the stream velocity and the position of maximum turbulence change, the banks of the stream undergo differential erosion which leads to meandering.
The maximum flow and erosion occurs on the outside of the meander while minimum erosion and deposition occur on the inside of the meander where point bars form. A helical flow, which develops across the channel cross section, transports material off the bottom upward to be deposited on the inside of the meander loop. The resulting deposits tend to be somewhat loose. During the high water stage, the path of maximum turbulence may follow a straighter course through the meander loops.
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