Streams that transport much of their load in suspension generally move in sweeping bends called meanders. These streams flow in relatively deep, smooth channels and primarily transport mud (silt and clay), sand, and occasionally fine gravel. The lower Mississippi River exhibits this type of channel.
Meandering channels evolve over time as individual bends migrate across the floodplain. Most of the erosion is focused at the outside of the meander, where velocity and turbulence are greatest. In time, the outside bank is undermined, especially during periods of high water. Because the outside of a meander is a zone of active erosion, it is often referred to as a cut bank (Figure 1).
Debris acquired by the stream at the cut bank moves downstream, where the coarser material is generally deposited as point bars on the insides of bends.
In this manner, meanders migrate laterally by eroding the outside of the bends and depositing sediment on the inside, moving sideways without appreciably changing their shape.
In addition to migrating laterally, the bends in a channel also migrate down the valley. This occurs because erosion is more effective on the downstream (downslope) side of the meander. Sometimes the downstream migration of a meander is slowed when it reaches a more resistant bank material. This allows the next meander upstream to gradually erode the material between the two meanders, as shown in Figure 2.
Eventually, the river may erode through the narrow neck of land, forming a new, shorter channel segment called a cutoff. Because of its shape, the abandoned bend is called
an oxbow lake.