Sediment driven meander migration in the Amazon Basin

Friday, 18 December 2015
Poster Hall (Moscone South)
Joshua Ahmed1, Jose A Constantine1 and Thomas Dunne2, (1)Cardiff University, School of Earth and Ocean Sciences, Cardiff, United Kingdom, (2)University of California Santa Barbara, Santa Barbara, CA, United States
Meander migration is the principal process controlling how river channels lengthen through time; despite this universal observation, little analysis has been dedicated to evaluating the mechanisms by which meanders accomplish this lengthening. Using an almost yearly archive of Landsat imagery, we show that meandering rivers in the Andes-Foreland Basin of the Amazon Basin lengthen linearly with centreline migration rate, in the absence of cutoff events. We characterised the dominant meander movement mechanism by defining an index we term the symmetry index. The index measures the ratio between downstream and upstream meander erosion about the apex and bounded by inflection points. Indices greater than one represent more translational meander deformation, that is, downstream migration, whereas indices close to one indicate more extensional migration (i.e., increasing meander amplitude).

We expanded our dataset to 25 reaches from varying physiographic provinces across the basin. Our results suggest that rivers located in sediment-rich regions migrate more rapidly, and possess higher symmetry indices indicative of more translational bend development. Conversely, rivers with low sediment yields show more extensional bend development. Since alluvial material is responsible for the construction of point bars, rivers conveying larger sediment fluxes have the ability to build bars more quickly. Point bar growth increases channel curvature and deflects high-velocity fluid towards the outer bank encouraging bank erosion. An analysis of point bar locations along the banks of two meandering streams shows that bars positioned downstream of the apex correlate with bends that undergo translational development, whilst material deposited in the centre and upstream of the apex show more extensional and lobing evolution. These results suggest that point bar growth and its relationship to the sediment budget of rivers play an important role in meander migration.