Streams meander to maintain equilibrium — a dynamically stable form and function. Remember, streams are transport machines, moving water and sediment from their watersheds relentlessly downslope. Meanders are produced when water in the stream channel erodes the sediments of an outer bend of a streambank and deposits this and other sediment on subsequent inner bends downstream.
This process reinforces the riffle-pool structure of a stream. As every angler knows, deep pools form on the outer bends of meanders, with low sloping point bars of accumulated gravel or cobble on the inside bends. In these pool and bend sections, one side of the channel is significantly deeper. Further downstream, moving out of the bend, the channel straightens, and the channel cross-section becomes more uniform through the riffle.
Support our Natural Stream Restoration efforts to recreate stable channels based on the hydrology and hydraulics that shape natural channels. Give with confidence today! Erosion in a stable stream can be minimal from year to year, but as a meander moves outward and becomes more looped, flows intensify on the outer bends.
Eventually, the meander may be cut off from the main channel, forming an oxbow lake. Oxbows along the West Branch Ausable River near Lake Placid, along River Road, are easily visible on Google satellite views; they indicate the river's former path prior to the Ausable's logging era. Meander patterns follow a remarkably predictable geometry described by wavelength, radius of curvature, and channel bankfull width — the height on the bank where, at higher flows, water begins to tip onto the floodplain see figure below.
No matter the size of the stream, the wavelength is approximately 11 times the channel width and, invariably, between 10 and 14 times the width. This area is referred to as the bank of the channel, and it is where erosion of sediment occurs. Erosion is caused by the increase in flow velocity, which has the force to erode and transport sediment from the bank. On the inside bend of the river, known as the point bar , water velocity is slower because it doesn't have to travel as far as the water on the outside of the bend.
This allows sediment to settle out and be deposited. As this cycle of erosion and deposition continues, the river will migrate in it's river valley. Due to the higher velocity in the middle and outside bend of the river channel, coarser sediment will be deposited in those areas.
The coarser grains will be transported to these parts of the river channel because the higher velocity is able to transport the heaver grains. This is related to the concept of Reynolds Number , which measures whether sediment transport of a specific grain size will be able to occur based on the velocity of the water. There are two distinct areas within the river channel that are characterized by different flow speeds and grain size, the outer edge and the inner bend of the channel.
The different flow speeds will carry different sized sediment grains, which will produce a cross section that is asymmetrical. The coarser grains will be present in the outer edges of the channel, while finer sediment will be along the inside of the channel. The different flow speeds within the channel, along with the different grain sizes, will produce unique sedimentary structures. The coarser grained areas that result from a faster flow speed will have upper planar lamination or dune cross stratification.
Meanders may cut into each other as they grow neck cutoffs , and then the river shortcuts growing meanders reduce the slope, cutoffs are a means to increase the slope again, feedback loop and the old meander is abandoned and slowly fills with fine sediment during floods oxbow lakes. Also, as a river builds up its levees and raises itself over the floodplain, the slope and the transport power of the stream decrease, the channel fills gradually with sediment, and finally often during a flood the river will breach its levee this process is called avulsion and follow a steeper path down the valley.
How meanders grow laterally through erosion outside bend and sediment deposition inside bend, point bar. When the loops get too large and consume too much energy friction , the river will eventually find a less energetically "taxing" shortcut, and a part of the old channel will be abandoned and becomes an oxbow lake. Air photos of meandering streams.
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