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. Because increasing climatic variability and anthropicpressures have affected the sediment dynamics of large tropical rivers,long-term sediment concentration series have become crucial forunderstanding the related socioeconomic and environmental impacts. Foroperational and cost rationalization purposes, index concentrations areoften sampled in the flow and used as a surrogate of the cross-sectionalaverage concentration. However, in large rivers where suspended sands areresponsible for vertical concentration gradients, this index method caninduce large uncertainties in the matter fluxes. Assuming that physical laws describing the suspension of grains in turbulentflow are valid for large rivers, a simple formulation is derived to model theratio ( α ) between the depth-averaged and index concentrations. The modelis validated using an exceptional dataset (1330 water samples, 249concentration profiles, 88 particle size distributions and 494 dischargemeasurements) that was collected between 2010 and 2017 in the Amazonianforeland. The α prediction requires the estimation of the Rousenumber ( P ), which summarizes the balance between the suspended particlesettling and the turbulent lift, weighted by the ratio of sediment to eddydiffusivity ( β ). Two particle size groups, fine sediments and sand,were considered to evaluate P . Discrepancies were observed between theevaluated and measured P , which were attributed to biases related to thesettling and shear velocities estimations, but also to diffusivity ratios β≠1 . An empirical expression taking these biases into account wasthen formulated to predict accurate estimates of β , then P ( Δ P = ± 0.03 ) and finally α . The proposed model is a powerful tool for optimizing the concentrationsampling. It allows for detailed uncertainty analysis on the averageconcentration derived from an index method. Finally, this model couldlikely be coupled with remote sensing and hydrological modeling to serve asa step toward the development of an integrated approach for assessingsediment fluxes in poorly monitored basins.

An index concentration method for suspended load monitoring in large rivers of the Amazonian foreland