Sediment-triggered migration and floodplain habitat development in meandering rivers

Meandering rivers create some of the most intricate and diverse regions on the planet. Lateral migration excavates floodplain material from the outer banks of meanders and subsequently deposits it at the hydrodynamically calmer inner bank region, creating point bars. Point bars are constructed by the progressive attachment of sediment in the form of scroll bar deposits: sediment shoals up and becomes deposited on the margins of the bar, increasing its aerial extent, which becomes stabilised by vegetation once regular inundation ceases. Point bars have the potential to affect flow as it is routed through the meander by both increasing the curvature of the bend and topographically forcing water to flow outwards, increasing boundary shear stress and rates of bank erosion. \nA combination of remotely-sensed data, GIS, and a 2D morphodynamic flow model was used to examine the role of externally-imposed sediment supply on point bar growth and floodplain evolution in the Amazon Basin. Externally-derived sediment supply is important since it provides the material used to build point bars – of course supplemented by internal sediment sources. A simulated reach on the Sacramento River, USA was used to investigate the role of increased sediment supply on bar growth and meander evolution through time. \nIt is demonstrated that rivers characterised by high sediment loads have greter migration rates, rates of cutoff production, and larger populations of oxbow lakes driven by the maintenance of a steady-state sinuosity on the rivers through time. Channel sinuosity increases with migration rate, although the rate of sinuosity increase is determined by the type of meander deformation: downstream translating bends increase their length more quickly than their upstream translating or extensional counterparts. Point bar growth was observed to occur under high sediment loading conditions when modelled using a 2D morphodynamic model. The bar sequestered sediment at the upstream head of the bar causing it to grow \nii \nupstream. This increased the distance of outer bank subjected to bank erosion and also increased to magnitude of bank erosion. a sediment-driven control on sinuosity increases manifested through bend deformation style, and the simulated growth of point bars in the presence of enhanced sediment loading which resulted in increased rates of bank erosion. These results are of significance for meandering theory and particularly indicate the importance of point bars in effecting the morphodynamic evolution of meandering rivers.