Fluvial sediment transfer in the Changjiang (Yangtze) river-estuary depositional system

Dam construction comes with severe social, economic and ecological impacts. From an ecological point of view, habitat types are altered and biodiversity is lost. Thus, to identify areas that deserve major attention for conservation, existing and planned locations for (hydropower) dams were overlapped, at global extent, with the contemporary distribution of freshwater megafauna species with consideration of their respective threat status. Hydropower development will disproportionately impact areas of high freshwater megafauna richness in South America, South and East Asia, and the Balkan region. Sub-catchments with a high share of threatened species are considered to be most vulnerable; these are located in Central America, Southeast Asia and in the regions of the Black and Caspian Sea. Based on this approach, planned dam locations are classified according to their potential impact on freshwater megafauna species at different spatial scales, attention to potential conflicts between climate mitigation and biodiversity conservation are highlighted, and priorities for freshwater management are recommended.

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Future large hydropower dams impact global freshwater megafauna.

Monitoring river channel dynamics using remote sensing and GIS techniques

We investigated the effects of two river restoration projects on hydromorphology and macroinvertebrate fauna in two German lowland rivers, the Schwalm and the Gartroper Mühlenbach. The stream channels were re-meandered and the floodplain levels were lowered to better connect the streams to their floodplains. The restoration was performed 10 years ago in the Schwalm and 2 years ago in the Gartroper Mühlenbach. We compared the restored reaches to nearby anthropogenically straightened reaches. Twenty-five hydromorphological parameters were recorded on twenty transects; between nine and 23 substrate-specific macroinvertebrate samples were taken per reach. Several hydromorphological parameters, such as the number and width of channel features and the number of substrate types, were significantly higher in the restored reaches compared to nearby anthropogenically straightened reaches. Total numbers of invertebrate families, genera, and taxa were also higher in the restored reaches than in the anthropogenically straightened reaches. Biotic substrates like dead wood or macrophytes were more abundant in the restored reaches, and these substrates hosted 28 taxa not found in the straightened reaches. While diversity was high in both restored reaches, overall abundance increased only in the river that was restored 10 years ago. Using NMS-analysis, substrate-specific faunistic samples of the restored reaches were compared to those of the straightened reaches. Our results revealed different invertebrate communities on the same substrates in the recently restored river. In the 10-year-old restoration, however, the same substrates were similarly inhabited. This comparable colonization of substrates may reflect succession in the macroinvertebrate community. The results are discussed according to the re-colonization potentials of the upstream and downstream reaches and the dispersal capacity of taxa.

Re-meandering German lowland streams - qualitative and quantitative effects of restoration measures on hydromorphology and benthic invertebrates

The lowermost Mississippi River (LMR) is important to the environment and economy of continental United States. Although bathymetric data have been collected over many decades at numerous c...

https://tandfonline.com/doi/pdf/10.1080/19475683.2019.1588781

Comparison of different spatial interpolation methods for historical hydrographic data of the lowermost Mississippi River

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/m3 for 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.

/pdf/riverbed-erosion-of-the-final-565-kilometers-of-the-yangtze-1jkn6fnqux.pdf

Riverbed erosion of the final 565 kilometers of the Yangtze River (Changjiang) following construction of the Three Gorges Dam

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2017JF004542

Decadal‐Scale Riverbed Deformation and Sand Budget of the Last 500 km of the Mississippi River: Insights Into Natural and River Engineering Effects on a Large Alluvial River

Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015

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.

https://www.mdpi.com/2073-4441/7/11/6079/pdf

Bo Wang

Papers

Riverbed erosion of the final 565 kilometers of the Yangtze River (Changjiang) following construction of the Three Gorges Dam

Decadal‐Scale Riverbed Deformation and Sand Budget of the Last 500 km of the Mississippi River: Insights Into Natural and River Engineering Effects on a Large Alluvial River

Dynamics of 30 large channel bars in the Lower Mississippi River in response to river engineering from 1985 to 2015

Sediment Trapping by Emerged Channel Bars in the Lowermost Mississippi River during a Major Flood

Long-term geomorphic response to flow regulation in a 10-km reach downstream of the Mississippi–Atchafalaya River diversion