Reversal of the sediment load increase in the Amazon basin influenced by divergent trends of sediment transport from the Solimoes and Madeira Rivers

The Amazon River is the largest river in the world in terms of freshwater discharge and sediment load. Understanding the fluvial sediment transport processes operating in the Amazon basin is critical because the Amazon basin has great research value for the sustainable development of local and global ecosystems. This study investigated changes in the mainstream sediment flux in the Amazon River system during 1996-2018. At the Obidos gauge station (on the Amazon mainstream), sediment loading significantly increased during 1996-2007 (earlier half of the study period), but the trend then reversed and sediment loading decreased during 2007-2018 (later period). River runoff was relatively stable during 1996-2018. The monthly sediment load and sediment concentration at Obidos were not synchronized with monthly runoff because of the asynchronism of the peak water discharges of the Solimoes, Madeira, and Negro Rivers. We infer that the significant increase in sediment flux from the Solimoes River was an important factor affecting the observed sediment increase at Obidos during 1996-2007. Besides, the increasing wash-load sediment flux at Manacapuru (on the Amazon tributary) should be derived from the Middle and Lower Solimoes River, because the sediment transport from the Upper Solimoes River was decreasing. We also found that the reversal of the sediment load trend at Obidos could be partly influenced by the decreasing trend of sediment transport from the Madeira River, and dam construction was one of the important causes. The reduced sediment loading in the Amazon mainstream will have major implications for the downstream ecosystem. Because planned dam constructions are expected to quadruple the number of dams in the Amazon basin, we suggest that the risk of wetland and muddy coast degradation caused by intensive hydropower development is an important pending issue.