Delayed neck cutoff in the meandering Black River of the Qinghai-Tibet plateau

Neck cutoffs in meandering rivers have long been thought to occur when the neck width (b) approximates the mean width of the parent channel (W). Empirical evidence for this paradigm is scarce, however, because tracking late-stage evolution of meander bends prior to cutoff at sufficient temporal resolutions is difficult in natural rivers. In this study, we captured field and photogrammetric data depicting temporal changes of the banklines forming the neck of one meander bend, and values of the narrowest neck width for 14 highly sinuous bends, in the Black River of the Qinghai-Tibet Plateau (China) over nearly four decades. Results show that the duration of bend evolution from the classic morphological threshold (b approximate to W) to the occurrence of neck cutoff could range from 52 to 161 years. This long period has hitherto been ignored by fluvial geomorphologists, and needs to be included in forthcoming kinematic and hydrodynamic models of meander evolution. Further analyses indicate that the neck narrowing process is autogenous at the local bend scale, neither controlled by the maximum bend sinuosity nor by the annual peak discharge.