Nonlocal effects of local cutoff disturbances along the meandering Ucayali River in Peru

Abstract:

For most migrating rivers, the time scale from bend initiation to cutoff is much greater than the period of recorded observations from aerial photography, historic maps, or field surveys. Efforts to elucidate meandering channel dynamics from such records have been further stymied by the poor time and spatial resolutions of the available data. The highly-active Ucayali River in Peru provides a unique opportunity to reveal natural, well-resolved meandering channel dynamics. In the past three decades alone, the Ucayali River has undergone many cutoffs including some bends growing from inception (nearly straight sections of river) until cutoff.

Annual binarized water masks of the Ucayali River were generated from over 30 years of Landsat imagery. A nearly-automated methodology employing image-processing techniques computes pixel-resolution banklines, centerline, width, curvature, and migration rate along hundreds of kilometers of river. This methodology together with the rapid migration of the Ucayali River captures the effects of flow, bar size, tributaries, and cutoffs on local migration rates. In this study, we document how cutoff-generated local perturbations in slope and sediment storage induces nonlocal accelerated meander migration both upstream and downstream. This process introduces a feedback through which amplified migration rates increase cutoff rates while cutoffs amplify migration rates. We also relate the magnitude of cutoff reaches to the spatial and temporal extents of their corresponding accelerated migration rates.