Ameander cutoff is a natural form of acutting or cut in a river occurs when a pronouncedmeander (hook) in ariver is breached by a flow that connects the two closest parts of the hook to form a new channel, a full loop. The steeper drop in gradient (slope) causes the river flow gradually to abandon the meander which will silt up with sediment fromdeposition. Cutoffs are a natural part of the evolution of a meandering river. Rivers form meanders as they flow laterally downstream (seesinuosity).[1]
Meandering rivers flow higher and hence with more total flow, pressure and erosion on the outside of their bends due to forming avortex as in a stirred coffee cup and consequently the river erodes more the outer bank. On the inside bend of a river, the level is lower,secondary flow moves sand and gravel across the river bed creating shallows andpoint bars, and friction of air and perturbances of the bed act against a higher proportion of the column of water, being shorter, slowing the water to varying degrees.[2] Rivers are commonly described and interpreted by their sinuosity. The term is equally used to describe the actual incidence of and potential tendency of a river to curve or meander over its length.[3] It is expressed as the ratio of the distance between two distant points in a river following the middle-of-the-river course of the river as compared with the straight distance between those points.[4] Three conventional categorizations of rivers or their reaches exist. Meandering rivers have a sinuosity value/ratio of greater than 1.5. A sinuosity value of less than 1.1 is a “straight” river. Between these values, a river is described as sinuous which describes those in a transitory state between the two states.Braided rivers do not follow this same convention.[3] Meandering rivers trend in the direction of increasing sinuosity.[5]
A river constantly evolves and as it does, meanders that were once a part of the river are abandoned in favor of a route that is more efficient for a river to take. As these old meanders are cutoff from the rest of the river, a new channel, or cutoff channel, is formed.
A chute cutoff channel can form during aflood resulting in an overbank flow where water goes over the banks of the river, creatingerosion of the surrounding landscape. More studies need to be done on how the magnitude of these floods and theirrecurrence interval are related to how often these chute cutoff channels form.[6] Neck cutoff channels are commonly formed the same way when an overbank flow occurs during a flood and the narrow piece of land between a bend in a meander is eroded away; this is known as rush-cutting.[5] A meander can also be cutoff by a channel due to excesssediment upstream as a result of high erosion rates. This leads to a cutoff channel forming since a river might no longer be able to carry that sediment through the bend efficiently, so the river forms a new path for it to flow.[7] Meander cutoffs can also be formed by humans; by removing abeaver dam, the likelihood that meander cutoff channels will be formed downstream increases.[7] A cutoff channel can beengineered for the purpose ofnavigation, traditionally forwater millleats and for controlling the possibility of any future flood were done on the lower reaches away from the tide. These meander cutoffs straighten a river.[8] Many rivers are transformed by humans, becoming less sinuous.[6]
When either of these meander cutoff processes takes place a bend of the river is left behind forming, in many instances, an oxbow lake. Anoxbow lake forms after there has beendeposition ofsediment, by the new cutoff channel flowing adjacent to it, at the entrances of the abandoned bend; this seals the bend off from the rest of the river. Oxbow lakes have been shown to be an importanthabitat for various species ofwildlife. Recent efforts have been made to protect these important bodies of water from harmful practices such asagricultural use.[8] One proposed method to restore these oxbow lakes has beendredging. Dredging will remove sediment from the lake’s floor and will increase the lake’s depth. Collins Lake inScotia, New York is an example of this method. Oxbow lakes can be valuable for recreational purposes and inSalix, Iowa, Browns Lake’s water level was increased forrecreational use.[8]
On 7 March 1876 a cutoff formed suddenly across the neck of a meander, known as the "Devil's Elbow", in theMississippi River nearReverie, Tennessee, shortening the river's course and leaving the town connected toArkansas, but across the new river channel from the rest of Tennessee.[9]
Some research has been done to show importance. Cutoffs have been shown to limit the age of a river meander and thus how large that meander can get, without which in areas of extreme sinuosity and low gradients means very long, slightly slowed, sections of rivers intensifying local flood risk.[10] Meander cutoffs influence the formation of a river’sfloodplain and continue to do so as the river evolves.[11] Cutoffs can affect the way that other river bends adjacent evolve over time, increasing their height of flooding and the direct momentum of the water which can then create further cutoffs and affect other bends further downstream throughout the river.[1] Meander cutoffs directly reduce and tend to indirectly reduce a river’ssinuosity, thus straightening out a river’s channel.[5] Understanding the processes that form meander cutoffs can allow one to predict how a river will evolve in the future which is important for agricultural businesses and controlling future floods.[5]
| Large-scale features | |
|---|---|
| Alluvial rivers | |
| Bedrock river | |
| Bedforms | |
| Regional processes | |
| Mechanics | |
