The world’s largest hydropower dam, the Three Gorges Dam (TGD), spans the upper Yangtze River in China, creating a 660-km long and 1.1-km wide reservoir upstream. Several recent studies reported a considerable decline in sediment load of the Lowermost Yangtze River (LmYR) and a rapid erosion in the subaqueous delta of the river mouth after the closure of the TGD in 2003. However, it is unknown if the TGD construction has also affected river channel and bed formation of the LmYR. In this study, we compared bathymetric data of the last 565 kilometers of the Yangtze River’s channel between 1998 and 2013. We found severe channel erosion following the TGD closure, with local riverbed erosion up to 10 m deep. The total volume of net erosion from the 565-km channel amounted to 1.85 billion m³, an equivalent of 2.59 billion metric tons of sediment, assuming a bulk density of 1.4 t/m³for the riverbed material. The largest erosion occurred in a 100-km reach close to the Yangtze River mouth, contributing up to 73% of the total net eroded channel volume.

Alluvial rivers are shaped by interactions of flow and sediment transport. Their lower reaches to the world's oceans are highly dynamic, often presenting engineering and management challenges. Here we analyzed over 6,000 single‐beam cross‐sectional measurements surveyed in 1992, 2004, and 2013 in the last 500‐km reach of the highly engineered Mississippi River, also known as the lowermost Mississippi River or LmMR, starting from the river's Gulf outlet to its avulsion into the Atchafalaya River. We applied inverse distance weighted interpolation to downscale the survey records into 10 × 10 m digital elevation models. We assessed riverbed deformation from bank to bank and quantified changes in riverbed sediment volume. The goal of our study is to test the hypothesis that the lower reach of a large alluvial river can function as a conduit for sediment transport under the current engineering focus of navigation safety and flood control. Our analysis shows that in the past two decades, nearly 70% of the riverine sand was trapped within the LmMR, and continuous riverbed aggradation occurred below the Mississippi‐Atchafalaya diversion, presenting favorable backwater conditions for avulsion. Backwater effects have mainly controlled riverbed deformation in the LmMR, while flow reduction may have also contributed to the channel aggradation in the uppermost and lowermost reaches. The study reveals the considerable complexity of geomorphic responses of a large alluvial river to human interventions, strongly suggesting that future river engineering and management of the world's alluvial rivers should focus on strategies and solutions that will improve sediment transport.

The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR). However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m². Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m³. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.

A recent study reported considerable sediment trapping by three large channel bars downstream 18–28km of the Mississippi–Atchafalaya River diversion (commonly known as the Old River Control Structure, ORCS) during the 2011 Mississippi River flood. In this study, we analyzed 3-decadal morphological changes of the 10-km river channel and the three bars to elucidate the long-term effects of river engineering including diversion, revetment and dike constructions. Satellite images captured between 1985 and 2015 in approximate 5-year intervals were selected to estimate the change of channel morphology and bar surface area. The images were chosen based on river stage heights at the time when they were captured to exclude the temporal water height effect on channel and bar morphology. Using a set of the satellite images captured during the period of 1984–1986 and of 2013–2014, we developed rating curves of emerged bar surface area with the corresponding river stage height for determining the change in bar volume from 1985 to 2013. Two of the three bars have grown substantially in the past 30 years, while one bar has become braided and its surface area has shrunken. As a whole, there were a net gain of 4,107,000m2 in surface area and a net gain of 30,271,000m3 in volume, an equivalent of approximately 36 million metric tons of sediment assuming a bulk density of 1.2t/m3. Sediment trapping on the bars was prevalent during the spring floods, especially during the period of 1990–1995 and of 2007–2011 when large floods occurred. The results suggest that although revetments and dikes have largely changed the morphology of the channel and the bars, they seem to have a limited impact on the overwhelming trend of sediment deposition caused by the river diversion