Depth-integrated suspended sediment and geochemical fluxes in large rivers: the Amazon River system

Erosion and weathering produce a wide range of residual solid products, in terms of size, density, mineralogy and chemical composition. These solid products are then transported by rivers from the continents to the oceans as suspended particulate matter (SPM) and bedload. Large rivers account for an important part of this transfer of sediments at the global scale. In those rivers, deep channels allow for vertical differentiation, or sorting, of suspended sediment, following their size and density. This hydrodynamic sorting results in vertically heterogeneous depth-profiles in terms of SPM concentration and size distribution (e.g. Garcia, 2008), which in turn likely result in an heterogeneous chemical composition of SPM throughout channel depth (e.g. Galy, 2007), which has to be evaluated. We sampled river water of the main tributaries of the Amazon River system (in the lowland basin), at two distinct water-stages, at various depths following depth-profiles, using a point depth-sampler. After filtration, and SPM recovery, SPM concentration, grain size distribution and chemical composition were determined. River discharge and water velocity throughout the sampled cross-sections were recorded using Acoustic Doppler Current Profiler (ADCP). The large increase in SPM concentration with depth observed at most of the sampled depth-profiles is well accounted for by the Rouse model (e.g. Rouse, 1950). This analysis allows us to reliably infer the SPM concentration and grain size distribution throughout the sampled river cross-section, and thus to estimate the spatially-integrated instantaneous SPM flux using ADCP data (Bouchez et al., 2010). The study also emphasizes the potential role of particle aggregation, within the river system, as a complicating factor regarding the prediction of these depth-integrated SPM fluxes from easily measurable hydrodynamic parameters (surface SPM concentration and grain size, and water velocity). Then, using the previous analysis, combined with the strong correlation between grain size parameters and chemical composition of suspended sediment observed in our set of samples, we deduce the chemical composition of SPM throughout the cross-section, and thus the spatially-integrated chemical fluxes transported as suspended load for a number of elements. These results are finally compared with “simple” estimates relying on the geochemistry of channel surface suspended sediments. This highlights the different behaviors of chemical elements with respect to hydrodynamic sorting in large rivers. References: M.H. Garcia, 2008, Sedimentation engineering: processes, measurements, modeling and practice, EWRI. V. Galy et al., 2007, Nature 450 : 407-411. H. Rouse, 1950, Engineering Hydraulics, Wiley, New York. J. Bouchez et al., 2010, Hydrol. Proc., accepted.