When a river flows from its source to its mouth, it often travels in meanders. A meander is a sinuous curve that is characteristic of the course. In general, rivers contain several bends where the river makes sharp turns and curves back on itself.
When water levels are high, erosion can take place on top of these bends and lead to new bends being formed when the erosion takes place in one area it will move to another.
The effects of erosion on meandering rivers occur due to sediment loads. As the river flows, it carries sediment in its currents.
Much of this sediment ends up in the middle of the river where it is carried out to sea and deposited along the coastline.
As a result of the erosion, meanders may be formed at the ends of bends and new bends could appear in between the existing bends.
The creation of new bends is most likely observed when materials are lost from a stream or when temporary channel cut-offs occur when high water volumes make a point susceptible to erosion and formation of new channels near that point.
Here are some more points discussed about Meandering Streams –
1. Sources of erosion.
Erosion occurs in a meandering stream due to several factors. The river flows in an erosive manner, which causes its banks to be eroded and the underlying bedrock is worn away.
Many meandering rivers have steep banks and are surrounded by hard rock such as granite or gneiss. These rivers erode their bottoms quickly.
Where erosion is taking place, the river deposits its sediments along the outside bend or bank. As mentioned above, meander bends will continue to evolve once a new one has formed or when one is eroded away.
There are two types of deposition: aggradation (sediment deposition) and degradation (sediment removal). Aggradation is characterized by sediment deposition at the downstream end of a meander bend, degradation by sediment removal at the upstream end of a meander bend.
The rate of erosion at a bend depends on whether there is an upstream or a downstream erosion edge, and on whether there are open water channels.
3. Erosion rates and sediment quantities.
The total amount of sediments carried by a meandering river increases with its discharge rate, which has been shown to increase with the square root of the slope gradient for rivers with sinuosity less than 0.5 (Li et al., 2005).
The total volume of sediments carried by a meandering stream is determined more by the volume of flow rather than its discharge rate.
4. Meander amplitude.
The meander wavelength and amplitude are inversely proportional to river sinuosity; i.e., sinuosity decreases with increasing amplitude and wavelength (You et al., 2005).
The total length of meanders along a river’s length is related to the stream discharge, so that as discharge increases, meanders become shorter in length for a given river’s sinuosity (Li et al., 2005).
5. Local variations in slope and curvature along rivers.
Local variations in curvature and slope have been found to have a significant influence on erosion. The more sinuosity, the greater is the influence of such local variations (Li et al., 2005).
For example, if the meander length increases from 10 km to 20 km downstream, then erosion becomes approximately four times as high at the upstream end. At the rotating end of a river bend, erosion can be twice as great as it is in between these two ends.
7. Frequency of meandering bends.
Turns of 2 degrees are common in braided streams having sinuosity under 0.5 (Li et al., 2005). It has been shown that for sinuosity 0.5 and higher, meandering has a frequency of roughly one turn per kilometre (Li et al., 2005).
For sinuosity 0.5 and higher, a single meander may flow for some distance before it is cut off by a new bend (You et al., 2005).
The rate of erosion depends primarily on the gradient of the channel. This can be expressed as the difference between velocity at the upstream and downstream ends of a meander.
8. Temporary channel cut-offs in the meandering bend.
The vertical difference between bed level and water surface is one of the most important parameters influencing flow and sediment transport. The greater this difference, the stronger will be the channel erosivity (Li et al., 2005).
Temporary channel cut-offs are observed when flood flows exceed a certain threshold value. The presence of a weak bed and of banks results in asymmetry of these flows and leads to formation of a temporary channel cut-off that is situated on the erosion side (downstream end) of the bend (You et al., 2005).