A meander forms when erosion wears away the outer bank of a river and sediment accumulates against its inner bank. Silt deposition during meander formation actually raises a river above the level of its floodplain.
A meander is a bend or loop in a waterway. Meanders form because water flows faster against a river's outer bank and slower along the inner bank. The faster flowing water erodes the outer bank, widening the channel and altering the flow of water. At the same time, the slower moving water along the inner bank carries silt and debris that accumulates at the shore. This combination of erosion and deposition creates a new channel for the river.
The deposition of silt during meander formation raises a river above the level of its floodplain, meaning that some water overflows the banks. In humid regions, the land around a meander remains under water constantly, creating a marsh or swamp.
Meandering rivers constantly and gradually change course over time as the meanders grow wider. Extremely large meanders are not energetically efficient for water flow. After meanders reach a certain size, water naturally finds a more efficient shortcut, causing the river to straighten, turning the meander into an oxbow lake.