Suspended sediment in rivers plays a crucial role in maintaining estuarine deltaic wetlands. In recent decades, many rivers worldwide have experienced a decrease in sediment transport due to dam construction. Currently, the dynamics and driving factors of suspended sediment along global estuarine deltas have not been comprehensively assessed, and the response of estuarine suspended sediment to changes in river sediment transport remains unclear.
The Yellow River Delta bathed in sediment. (Image provided by Sun Yat-sen University)
A joint effort by Sun Yat-sen University, Southern University of Science and Technology, East China Normal University's Water Environment Remote Sensing Team, and Utrecht University's Coastal Morphodynamics Team has focused on the global variation of suspended sediment in river estuaries. Utilizing satellite remote sensing technology, they monitored changes in suspended sediment concentrations at the mouths of 349 major rivers worldwide over the past 20 years. This endeavor produced a monthly average dataset of suspended sediment concentrations for these 349 river mouths, analyzing how changes in suspended sediment relate to variations in sediment transport in rivers and identifying key factors influencing suspended sediment dynamics. On April 18, the findings were published in Nature Communications.
"This study contributes to understanding the spatiotemporal patterns of suspended sediment in river deltas, which is crucial for scientifically assessing the vulnerability of coastal wetlands under the background of sea-level rise. It also lays a scientific foundation for the formulation of relevant policies for coastal protection and restoration in various regions," said Associate Professor Hou Xuejiao from the School of Geodesy and Geomatics at Sun Yat-sen University, the first author of the paper, in an interview with China Science Daily.
Unveiling the Changes in River Estuarine Suspended Sediment
Coastal suspended sediment plays a critical role in maintaining coastal wetlands, coastal ecosystems, and the development of river deltas.
Many studies have shown that the construction of dams upstream of rivers has led to reduced suspended sediment in some river mouths (e.g., the Yangtze River in China and the Mekong River in Vietnam), resulting in issues such as coastal erosion and mangrove loss in river deltas. However, these studies have been limited to specific regions, and it remains unclear whether river estuaries worldwide are facing reduced suspended sediment due to dam construction.
"In recent decades, various global assessments have indicated that the construction of dams on rivers has led to reduced sediment transport in many rivers. However, a study in 2022 using radioactive isotopes examined sediment changes in nearshore areas of North America around 1950 and found that despite the sharp increase in dam construction after 1950, there was no reduction in sediment in North American nearshore areas; in fact, sediment volume more than doubled after dam construction. One possible explanation is that coastal suspended sediment is influenced by coastal hydrodynamics, such as feedback between river flow, tides, waves, and sediment transport. Therefore, understanding the long-term changes in suspended sediment in river estuaries and their influencing factors is crucial for accurately assessing the future evolution of river delta coastlines," explained Associate Professor Feng Lian from the School of Environmental Science and Engineering at Southern University of Science and Technology, the corresponding author of the paper.
To understand the changes in global river estuarine suspended sediment, the team used MODIS satellite remote sensing data to construct a dataset of suspended sediment from 2000 to 2020 for 349 river estuaries, providing the most extensive and detailed dataset of suspended sediment concentrations in river mouths to date. Their research found that over the past 20 years, most river estuaries globally have shown an increasing trend in suspended sediment, with significant increases observed in North America, Africa, and the Arctic, while a significant decrease was noted in river estuaries in South Asia.
Revealing the Response of River Estuarine Suspended Sediment to Sediment Transport
It is commonly believed that reduced sediment transport in rivers will limit the amount of suspended sediment in river estuaries, thereby increasing the risk of loss of coastal wetlands.
To understand the influence of sediment transport in rivers on suspended sediment in river estuaries, the team analyzed the response of suspended sediment in river estuaries to changes in sediment transport in rivers using satellite-derived data on changes in sediment transport in rivers combined with the obtained data on changes in suspended sediment in river estuaries. The study found that 45.2% of changes in suspended sediment in river estuaries were opposite to changes in sediment transport in rivers. Analysis revealed that these river estuaries had relatively low sediment transport in rivers and weak tidal effects but strong wave effects.
To further explore the reasons for the inconsistency in changes between suspended sediment in river estuaries and sediment transport in rivers, the team used satellite-derived data on sediment transport in rivers combined with model-simulated tidal, wave, and salinity data to analyze the factors influencing changes in suspended sediment in river estuaries. The study found that seasonal changes in suspended sediment in river estuaries were closely related not only to sediment transport in rivers but also to tidal and wave effects. Seasons with high suspended sediment concentrations were associated with high river sediment transport and strong tidal or wave effects. This finding highlights the complexity of the response of suspended sediment in river estuaries to sediment transport in rivers.
Interestingly, the study found that in the long term, river estuaries with high sediment transport and strong tidal effects tended to have higher suspended sediment concentrations, while those with strong wave effects tended to have lower suspended sediment concentrations.
"The occurrence of these differences in seasonal and long-term impacts may be due to the fact that changes in suspended sediment in river estuaries are not only influenced by sediment transport in rivers and coastal hydrodynamic factors but also by coastal geomorphology. Compared to river-dominated and tide-dominated river deltas, wave-dominated river deltas have lower suspended sediment concentrations. This may be because wave-dominated river deltas lack many distributary channels and mostly have sandy coastlines, characteristics that make it difficult for wave-dominated river deltas to effectively retain suspended sediment over the long term. Waves may temporarily increase suspended sediment concentrations, but long-term strong wave dominance often leads to decreased suspended sediment concentrations," explained Assistant Professor Jaap H. Nienhuis from Utrecht University, the corresponding author of the paper.
Future Applications
The changes in global river estuarine suspended sediment monitored in this study and their response to sediment transport in rivers can provide valuable theoretical support for the protection and restoration of coastal wetlands in river estuarine coastal areas. The study found that not all reductions in sediment transport in rivers lead to reductions in suspended sediment in river estuaries, at least within the time frame analyzed in this study. Furthermore, the study found a significant increase in suspended sediment in river estuaries over the past two decades. This increase in suspended sediment may help coastal wetlands in river estuaries better withstand the threat of future sea-level rise, which also suggests that previous studies may have overestimated the vulnerability of coastal wetlands. The dataset provided by this study can also serve as a data support for managing and restoring coastal environments. Increased suspended sediment promotes the expansion of delta areas, with abundant sediment forming new mudflats, providing more habitat for various organisms. However, not all increased suspended sediment has a positive effect on coastal ecosystems. Elevated suspended sediment concentrations can affect underwater photosynthesis and the survival of aquatic organisms, as increased turbidity reduces underwater light visibility. Moreover, increased suspended sediment may exacerbate water quality deterioration and even lead to extensive algae blooms, as suspended sediment serves as an important carrier of land-based pollutants such as pesticides, nutrients, and heavy metals. Therefore, further research is needed to determine whether the increase in estuarine suspended sediment is beneficial for the sustainable development of coastal zones.
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