LAUSR

We present the first spatiotemporal surface suspended sediment concentration (SSSC) analysis of the lower Amazon River, by combining geomorphological mapping, hydrosedimentological field investigations, and 15 years (2001–2015) of weekly averaged wash-load (fine sediment) fluxes at gauging stations estimated from field-calibrated remote-sensing models of SSSC. We found a downstream increase in the magnitude of sediment sinks, where the floodplain geomorphic style controls sediment trapping, and we conclude that the sediment net loss in the floodplain happens during the rising phase through seasonal hydrosedimentological connectivity. On average, 120 million tons per year of fine sediments are accumulated in the floodplain along 765 km of the lower Amazon River (from the confluence with the Madeira River to Monte Alegre city), making the lower Amazon River one of the most important fluvial sediment sinks among the world’s axial rivers. INTRODUCTION The floodplains of the largest axial rivers can act as continental sedimentary sinks by storing sediment and preserving sediment archives in the geological record (Latrubesse, 2015). However, it remains challenging, both scientifically and methodologically, to understand “where” the sediments are stored in the alluvial reach, “how” the sediments are transferred to the sinks, “what” landforms result by channel-floodplain morphodynamics, and “how much” sediment is stored. Here, we intend to contribute to the understanding of these issues by studying Earth’s largest axial river, the Amazon. Dunne et al. (1998) quantified deposition rates in the channel and the floodplain between São Paulo do Olivença and Obidos, Brazil. Filizola and Guyot (2009) calculated the annual and seasonal deposition of suspended sediments from the Madeira River confluence to Obidos, and Mangiarotti et al. (2013) assessed wash-load storage between Itacoatiara and Obidos. Others have produced local sediment budgets in Curuai Lake (Fig. 1; Bourgoin et al., 2007; Rudorff et al., 2017) but obtained contrasting results. Here, we updated and refined the lower Amazon floodplain sediment budgets using a new method that incorporates wash-load sediment fluxes and floodplain geomorphic styles (see the GSA Data Repository1). We analyzed the magnitude of sedimentation from Manacapuru to Monte Alegre (~1200 km) and identified the hotspots of sedimentation for muddy sediments (silt and clay), and we provide the first regional map of surface suspended sediment concentration (SSSC) of the lower Amazon River (Fig. 1). Beyond the specific geoscientific interest, spatial understanding and quantification of channel-floodplain interactions in the Amazon are also environmentally relevant. The sediment regime of the Amazon River remains largely unregulated, but the construction of more than 200 dams has been proposed in the Amazon Basin. Consequently, any alteration of the sediment regime of the Amazon’s floodplain, estuary, and sediment plume by dams is a major environmental concern (Latrubesse et al., 2017).