A meander is a winding curve or bend that develops in a river channel. This sinuous pattern is characteristic of mature river systems flowing across relatively flat terrain, such as a floodplain. Meanders represent the river’s dynamic attempt to find the path of least resistance, continually reshaping the landscape through the paired processes of erosion and deposition.
Visual Characteristics of a Meander
Meanders typically appear as a series of regular, S-shaped curves where the river snakes back and forth across a broad, low-gradient valley floor. They are most commonly found in the middle and lower courses of a river, where the primary form of channel change shifts from vertical downcutting to lateral erosion. The degree of the river’s curvature is quantified by its sinuosity, which is the ratio of the channel length to the straight-line distance down the valley. Rivers with a sinuosity value of 1.5 or greater are classified as meandering streams.
Meander Geometry
The overall geometry of a meander can be described using terms like amplitude and wavelength. Amplitude refers to the maximum lateral distance between the axis of the meander and the centerline of the valley, while wavelength is the distance between corresponding points on successive bends. The wavelength of a meander often correlates directly with the width of the channel, typically ranging from seven to ten times the channel width. The entire zone over which the river shifts is known as the meander belt, a width that can be 15 to 18 times the channel width.
The Process of Meander Development
As water flows around a curve, centrifugal force pushes the fastest-moving water toward the outer bank. This concentration of higher velocity at the outer curve results in intense erosion, primarily through hydraulic action and abrasion, leading to the formation of a steep, undercut feature called the cut bank.
Conversely, the water on the inner bank moves significantly slower due to reduced force and increased friction. This drop in velocity causes the river to lose its capacity to carry sediment, leading to the deposition of sand, gravel, and silt. This deposited material accumulates to form a gently sloping feature known as a point bar, which builds up on the inside of the bend. The continuous erosion on the outside and deposition on the inside causes the meander to migrate laterally across the floodplain.
A mechanism called helicoidal flow is the primary driver of this sediment transfer across the channel. This flow is a corkscrew-like, secondary current that moves water from the surface near the outer bank down to the bed, then across the channel floor toward the inner bank, before rising again near the surface. This spiral motion effectively sweeps eroded material from the cut bank and deposits it onto the point bar. This constant shifting creates a highly asymmetrical channel cross-section, with the deepest water, or thalweg, located near the eroding cut bank.
Associated River Features
The continuous lateral movement of meanders across the landscape creates several distinct landforms. As a meander’s curvature increases, the narrow piece of land separating two adjacent bends, known as the meander neck, becomes thinner. During a major flood event, the river may acquire enough energy to breach this narrow neck, taking a straighter and shorter path.
When this neck cutoff occurs, the main river flow bypasses the old meander loop. The loop is sealed off from the main channel by the deposition of sediment at both ends. The isolated, crescent-shaped body of water that remains is called an oxbow lake. Over time, these oxbow lakes are subject to natural infilling as they accumulate fine silt and organic material, eventually transforming into marshes.
Once an oxbow lake is filled with enough sediment, it leaves behind a subtle depression on the floodplain known as a meander scar. These scars are visible as curved lines or tracts of different vegetation, marking the former path of the river channel. The sweeping movement of meanders and their associated cutoff events are fundamental in building and maintaining the wide, flat expanse of the floodplain.