Coupure de méandre

A meander cutoff is a natural form of a cutting or cut in a river occurs when a pronounced meander (hook) in a river 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 from deposition. Cutoffs are a natural part of the evolution of a meandering river. Rivers form meanders as they flow laterally downstream, see sinuosity. Meandering rivers flow higher and hence with more total flow, pressure and erosion on the outside of their bends due to forming a vortex 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 and point 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. 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. 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. 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. Meandering rivers trend in the direction of increasing sinuosity. 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.