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• A mathematical model considering the operation of 24 mega reservoirs was proposed. • The sedimentation mainly occurs in the mainstream during the flood season. • The cascade reservoirs operation leads to easier and prolonged sedimentation. The intensive construction of large dams in the upper Yangtze River Basin (UYRB) has formed large-scale cascade reservoirs and obviously altered the flow regime and sediment transport of the basin. However, few studies have quantitatively described the cumulative impact of these reservoirs on sediment interception. In this study, a flow and sediment transport model that fully considers the operation processes of 24 mega reservoirs in the UYRB was established, and the spatial and temporal variations in sedimentation from 2011 to 2080 were elucidated, assuming repeated series of daily input flow discharge and sediment concentration during 2011–2020 and monthly water operation levels of 24 reservoirs during 2018–2021 as boundary conditions. The results showed an annual average sedimentation of more than 300 Mt in the UYRB during 2011–2060, which falls to 270 Mt in the period 2061–2080, and more than 80% of the sedimentation occurs in the flood season. Nearly 80% of the total sedimentation during 2010–2080 was observed in the mainstream reservoirs, especially the four mega reservoirs in the lower Jinsha River and the Three Gorges Reservoir (TGR), which accounted for 31.9% and 22.7% of the total sedimentation, respectively. The proportion of sedimentation in mainstream reservoirs will further increase with increasing operation periods. Most reservoirs in the UYRB will not reach sedimentation equilibrium by 2080, except for reservoirs in the Min River Basin. The ratio of siltation capacity to average annual sediment yield determines the percentage of residual siltation capacity of each reservoir. The operation of the cascade reservoirs in 2013–2020 had no more immediate impact on the middle and lower Yangtze River than that in 2003–2012, when the TGR started impoundment. However, the joint operation of these cascade reservoirs has changed the spatial distribution of sedimentation in the UYRB, and will lead to prolonged impacts on downstream channel erosion. [ABSTRACT FROM AUTHOR]

Debris flow can be very destructive, especially in areas of dense population on mountain foothills. Check dams are constructed across drainage channels and are beneficial for reducing flow velocity and soil erosion, as well as controlling debris flows. On July 26, 1987, a destructive debris flow occurred in a watershed north of Tehran, Iran. A series of check dams were constructed in the watershed to mitigate the hazards of future debris flow due to the importance of the region. This study first proposes a procedure to estimate the volume of the sediments in a debris flow from the calculated volume of an equivalent regular flood. The procedure to estimate the sediments volume is employed in a framework to assess the effects of constructed check dams for a similar debris flow in the region. The framework considers the unfilled storage of the check dams for estimating the volume of the sediments in each catchment. Field measurements were implemented to estimate the available storage of check dams. The volume of sediments in two cases was estimated: 1) the debris flow in 1987 and 2) a probable debris flow at current time considering the available storage of constructed check dams. The comparison shows that the volume of sediments reaching the outlet of the watershed decreased 13% as the effect of check dams in trapping sediments. Applying the proposed framework to this case study shows its ability in assessment of check dams' impact on debris flow. [ABSTRACT FROM AUTHOR]

• Reversal of sediment load trend between the earlier and later periods was observed. • Divergent trends of sediment transport from the tributaries influenced this reversal. • Dam construction was partly responsible for the recent reduction in sediment load. 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 Óbidos 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 Óbidos were not synchronized with monthly runoff because of the asynchronism of the peak water discharges of the Solimões, Madeira, and Negro Rivers. We infer that the significant increase in sediment flux from the Solimões River was an important factor affecting the observed sediment increase at Óbidos during 1996–2007. Besides, the increasing wash-load sediment flux at Manacapuru (on the Amazon tributary) should be derived from the Middle and Lower Solimões River, because the sediment transport from the Upper Solimões River was decreasing. We also found that the reversal of the sediment load trend at Óbidos 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. [ABSTRACT FROM AUTHOR]