Meander cutoff

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).^[1]

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.^[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]