Meanders are the characteristic bends within a river that are generally found in the middle
section of a river. They can vary in size, but generally have a deposit of various sized
material on the inside of the bend called a slip off slope or point bar, and an incised concave
section on the opposite bank of the river called a river cliff or undercut.
The initial cause of meandering rivers is related to the way in which pool and riffle
sequences cause changes in the hydraulic radius of a river which in turn causes changes to
the way in which the water moves within the channel. Indeed, rivers rarely have straight or
regular paths, as they seek to overcome variations in the Earths surface and seek out the
path of least resistance on their journey from source to mouth.
First, even in straight channels the river will deposit sediment in alternating bars that will
eventually create riffles. This will occur when there are low flows in the river, and the low
hydraulic radius (channel efficiency) is enough to encourage deposition. Once created, the
riffle will continue to lower the hydraulic radius for the area and water is seen to flow even
more inefficiently over it. Second, water needs to find a way around these areas of higher
frictional contact so it flows around them. This creates the variations in flow and introduces
a side to side motion to the water; this is effectively the start of meander development.
Between these shallow riffle areas deeper areas called pools are eroded, mainly at times of
higher discharge, so a series of pools and riffles develop over time. Next, at times of higher
flow the water will swing around one side of a riffle and this will undercut the opposite bank
through erosion processes because of centripetal force (it is swung outwards).
Once the slightly meandering course is established, it is exaggerated by the combined
processes of erosion and deposition. The outside of the bend has deeper water, hence a
higher hydraulic radius which means that the water moves more quickly free from frictional
drag. This fast flowing water erodes the bank through hydraulic action (where the sheer
force of the water erodes the stones, bed and banks of the river) and corrosion (where
stones in transport are thrown into the bed and the banks eroding them) causing
undercutting, bank collapse and the river will move laterally helping to widen a flood plain.
On the opposite side, a shallower profile will lower hydraulic radius (which means lower
efficiency) and hence sediment will settle out, creating a point bar or slip off slope. The
river current will swing around the outside of the bends at the surface and will swing back to
the opposite side as subsurface flow. This helicoidal flow allows eroded material from the
outer bank to be deposited on the next point bar.
In the future, meanders can eventually turn into Ox-bow lakes. As the meanders increase in
sinuosity they can create a narrow neck of land between meander loops. This neck of land
can be cut through in flood events, leaving the meander as an abandoned lake. The lake is
further cut off by deposition, and it will slowly infill with plants and sediment, and water will
slowly evaporate from it.