Multiscale flow-vegetation-sediment feedbacks in low-gradient landscapes

Low-gradient fluvial landscapes (1-50 cm km-1) occupy diverse settings (inland, tidal, and deltaic wetlands, large-river floodplains, lowland streams) but share many commonalities. Because of low flow energies, these landscapes have abundant vegetation that is submersed or frequently inundated. Their sediment loads are predominantly fine, with large loads transported in suspension. The complexities of fluvial-vegetation interactions and fine sediment transport have made these landscapes relatively poorly studied in the field of geomorphology, yet the importance of understanding their dynamics is paramount: these landscapes will be among the first inundated by rising seas and/or are presently the focus of the world's largest restoration projects. In this synthesis, interactions between flow, vegetation, and sediment documented in inland and tidal marshes, chalk and Coastal Plain streams, large river floodplains, and deltas are reviewed and classified into feedback processes occurring over a range of five spatiotemporal scales. Despite different morphologic appearances, the feedback processes governing the coevolution of landscapes and vegetation are broadly consistent across settings, though modified in their effects by life-history traits of vegetation, characteristics of the sediment supply, external boundary forcing, and initial conditions. Longitudinally elongated patches are common in many settings, arising from stress divergence feedbacks. However, their ultimate morphology is strongly dependent upon the activation of patch- or multipatch-scale feedbacks, which depends on initial densities at the time of colonization. Generally, flow vegetation-sediment feedbacks play a dominant role in landscape morphology when the setting is depositional and little initial topographic differentiation is present. In that case, these feedbacks can explain the apparently paradoxical emergence of high sinuosities or parallel drainage in very low gradients. Importantly for management, flow-vegetation-sediment interactions may activate a chain of positive feedbacks that engages progressively larger scales, resulting in massive destruction of marsh vegetation or topographic homogenization. Interrupting these feedbacks requires intervention over all scales activated. (C) 2019 Published by Elsevier B.V.