Riverine Sediments Research Articles

Evidence and Implications of Hydrological and Climatic Change in the Reno and Lamone River Basins and Related Coastal Areas (Emilia-Romagna, Northern Italy) over the Last Century

Climate change and human activities have consequences on coastal areas as they affect hydrological processes in the related river basins. The riverine sediment supply to the beaches of the Emilia-Romagna coast, a highly urbanized area with high economic and naturalistic value, has been heavily impacted by human activities throughout the catchment, reducing solid transport to the coast and increasing the threat of coastal erosion and flooding. Despite the introduction of safeguard policies in the early 1980s and the consequent stoppage of such activities, the expected return in solid transport has not yet been reflected at the coast. To better understand the various processes acting at the river basin scale, we utilized empirical mode decomposition to analyze the variability in different parameters (river discharge, rainfall, air temperature, and sea level) from the headwaters to the coast of the Reno and Lamone rivers over the last century. The anthropogenic footprint, linked to the large-scale dimming/brightening phenomenon, is visible in the long-term trends. Moreover, natural signals with variable periodicity are evident and partially correlated with two major climate modes (North Atlantic Oscillation and Atlantic Multidecadal Oscillation). The coupled interactions among these processes, combined with the changes in land use and evapotranspiration during the last century, have resulted in the prolonged scarcity of river sediment supply and a long-term trend of erosion of the coastal area.

Evolution of rare earth element and εNd compositions of Gulf of Mexico seawater during interaction with Mississippi River sediment

A closed-system batch reaction experiment was conducted for 270 days to evaluate the effects of interaction between Gulf of Mexico (GOM) seawater and Mississippi River sediments on the system’s dissolved rare earth elements (REE) concentrations and neodymium isotopic compositions (εNd). This study specifically focuses on geochemical reactions involving silicic sediments derived from weathering of the North American continent as they affect the REEs and εNd of seawater along continental margins, in contrast to previous studies that investigated the influence of basaltic rocks and sediments on REEs and εNd in the ocean. Our results show that the dissolution of labile phases of lithogenic Mississippi River sediments leads to an approximately 100-fold increase in dissolved REE concentrations within the first 33 days of the experiment. Secondary mineral precipitation appears to lower the REE concentrations between days 33 and 270 of the experiment, although seawater REE concentrations remain elevated compared to initial values. The two-way elemental transfer involving dissolution and precipitation results in a net increase by a factor of 24 ± 12 (mean ± 1σ) in the dissolved REE concentrations by the end of the experiment (i.e., day 270). The dissolved REE concentration maxima observed after 33 days of the experiment represent the mobilization of approximately 0.37 % of the REE content of the operationally defined “exchangeable” fraction of the riverine sediments. The εNd values of the reactive lithogenic components were −9.77 and −9.95, which are similar to the GOM value of −9.81 ± 0.36. Because of the similarity between εNd values, changes in the seawater Nd isotope value throughout the experiment were subtle (mean ± std, reacted seawater εNd of −9.87 ± 0.17). The highest REE concentrations coincided with the most radiogenic εNd (-9.65 ± 0.23; day 33), which suggests that REE concentrations and εNd compositions of the GOM may be buffered by fluxes from sediments in the system. Our results are comparable to previous studies involving basaltic rocks and/or sediments of basaltic composition in that they demonstrate that silicic, river sediments are highly reactive in marine environments with regard to REE mobilization. The experimental results further suggest that “boundary exchange” plays an important role in influencing the εNd of seawater along continental margins dominated by large river systems, although the impacts of boundary exchange will be most profound where ambient seawater and river sediments have distinct Nd isotopic compositions (e.g., basaltic, or Precambrian shield material). Finally, our results indicate that the εNd value of GOM seawater is largely controlled by the lithogenic sediment delivered to the basin by the Mississippi River.

Reconciling the geogenic and non-crustal origins of elements in an Indo-Bangla transboundary river, Atrai: Pollution status, sediment quality, and preliminary risk assessment

This study has been conducted on an anthropogenically less influenced transboundary river (Atrai: Indo-Bangladesh) to comprehend the inherent geochemistry and identify potential elemental sources. In doing so, across the Bangladeshi portion, 30 river-bed samples were culled and studied by neutron activation analysis to quantify the abundances of 15 geochemically and toxicologically significant elementals (Na, Al, K, Ti, Cr, Mn, Co, Zn, As, Rb, Sb, Cs, Ba, Th, and U). The results revealed that the mean concentrations (μg/g) of Rb (154.6), Cs (7.53), Th (20.90), and U (4.88) were 1.5–2.0 times higher than crustal values. Besides, geo-environmental indices revealed ‘uncontaminated to moderately contaminated’ pollution status with minor enrichment or contamination for Rb, Th, Sb, U, and Cs, relatively concentrated in the mid-to-downstream zone possessed geogenic and non-crustal origins. The positive matrix factorization and other statistical approaches revealed predominant geogenic enrichment of Na, K, Al, Ti, Zn, Cs, Rb, As, Th, and U from differential mineralogical compositions via weathering, elemental fractionations, and biogeochemical mobilization. Contrariwise, several anthropogenic sources (for Cr, Sb, Co, Mn, Th) were also ascertained in the vicinity of Atari River. However, sediment characterization based on SQG threshold values manifested that Cr and Mn possess rare biological effects on local aquatic organisms. Nevertheless, SQGs-based and ecological risk indices invoked minor to no potential ecotoxicological intimidations for the considered metal(oid)s (Cr, Mn, Co, Zn, As, and Sb). Hence, this study manifested the usefulness of a less anthropogenically affected river to reckon geogenic and non-crustal elemental origins in the compounded riverine sediment.

Pollution level of trace metals (As, Pb, Cr and Cd) in the sediment of Rupsha River, Bangladesh: Assessment of ecological and human health risks

The study area was selected in the Rupsha river basin and the sediment samples were collected to determine trace metal concentrations of As, Pb, Cd, and Cr along with biological effects, and potential ecological and human health hazards for adults and children. The concentrations of trace metals were detected from sixty composite sediment samples using an Atomic Absorption Spectrophotometer (AAS) following some sequential analytical procedures. The mean concentrations of trace metals were organized in the descending order of chromium (Cr) (43.2 mg/kg) > lead (Pb) (29.21 mg/kg) > arsenic (As) (5.18 mg/kg) > cadmium (Cd) (1.8 mg/kg). The current study highlighted that metals were attributed to the riverine sediments from natural sources and other anthropogenic sources, particularly from various industries. Based on the effect-range classifications of threshold effect concentration (TEC) and probable effect concentration (PEC), the trace metal concentrations can impact on the sediment-dwelling organisms occasionally especially for Cr, Cd, and Pb. That is, the concentrations had negative biological consequences on aquatic creatures. The assessed potential ecological risk of Cd offers a significant risk to the aquatic ecosystem, whereas As, Cr, and Cd were in low-risk. Most of the sites of the study area were within the range of moderate risk, indicated by the risk index (RI) values. Furthermore, the applied sediment quality indices, geo-accumulation index (Igeo) indicated that sediment was contaminated by Cd whereas contamination factor (CF) denoted that the sediment of the study area was moderately polluted by Pb. However, pollution load index (PLI) revealed that the study area was polluted for cumulative sence especially in winter season. The age-group risk index (HI) was much lower than the threshold limit of 1, showing that the pollution had no non-carcinogenic risk effect. Total carcinogenic risk (TCR) was less than one-tenth of a percentile. For the sake of human and environmental health, proper monitoring of metal element attribution and strict regulation are required to lessen trace metal pollution.

Impacts of large projects on the sediment dynamics and evolution of the Hengsha Shoal in the Yangtze Delta

Global deltas are facing severe challenges under the impacts of riverine sediment decline and large estuarine projects. Comparing with abundant research on the Yangtze subaqueous delta, the shoals in shallow waters are poorly understood. The Hengsha Shoal, Yangtze Delta, China, expands during the past ∼65 years, with only slight changes during the pre-projects period and remarkable variations in the projects period. It's now half-surrounded by the Hengsha Reclamation Project and the Deep Waterway Project, both facilitating the recent expansion. The current vertical net rate of change in the study area is −2.2 cm/yr, suggesting an overall balance between sediment deposition and erosion. Spatially, the Hengsha Shoal is currently changing from horizontal expansion to vertical accretion. Considering the spatial patterns of erosion/deposition and residual currents and sediment, modern geomorphic changes in the Hengsha Shoal appear to be dominated by internal sediment transport from deep to shallow waters. Considering further possible changing conditions, the future evolution of this shoal and the resulting potential ecological impacts should be studied further. This case study demonstrates that sediment dynamics in semi-artificial shoals can be totally different from the subaqueous delta, highlighting the importance of human engineering in governing large deltas globally.

Estimating the lateral transfer of organic carbon through the European river network using a land surface model

Abstract. Lateral carbon transport from soils to the ocean through rivers has been acknowledged as a key component of the global carbon cycle, but it is still neglected in most global land surface models (LSMs). Fluvial transport of dissolved organic carbon (DOC) and CO2 has been implemented in the ORCHIDEE LSM, while erosion-induced delivery of sediment and particulate organic carbon (POC) from land to river was implemented in another version of the model. Based on these two developments, we take the final step towards the full representation of biospheric carbon transport through the land–river continuum. The newly developed model, called ORCHIDEE-Clateral, simulates the complete lateral transport of water, sediment, POC, DOC, and CO2 from land to sea through the river network, the deposition of sediment and POC in the river channel and floodplains, and the decomposition of POC and DOC in transit. We parameterized and evaluated ORCHIDEE-Clateral using observation data in Europe. The model explains 94 %, 75 %, and 83 % of the spatial variations of observed riverine water discharges, bankfull water flows, and riverine sediment discharges in Europe, respectively. The simulated long-term average total organic carbon concentrations and DOC concentrations in river flows are comparable to the observations in major European rivers, although our model generally overestimates the seasonal variation of riverine organic carbon concentrations. Application of ORCHIDEE-Clateral for Europe reveals that the lateral carbon transfer affects land carbon dynamics in multiple ways, and omission of this process in LSMs may lead to an overestimation of 4.5 % in the simulated annual net terrestrial carbon uptake over Europe. Overall, this study presents a useful tool for simulating large-scale lateral carbon transfer and for predicting the feedbacks between lateral carbon transfer and future climate and land use changes.

Holocene evolution of the shelf mud deposits in the north-western South China Sea

Marine mud deposits contain rich information on past interactions between riverine sediment fluxes and marine processes. Massive mud deposits attached to a major river from South China, the Pearl River, are distributed on the north-western shelf of the South China Sea. This study examines the evolution history using cores penetrating through Holocene strata and deciphers its response to the river system, sea-level change, monsoon variations, human activities, etc. Geochemical and sedimentological data constrained by robust radiocarbon data show the difference in the evolution of mud deposits in shallow waters and on the middle shelf. Muddy wedges in shallow waters along the coast have formed since 7 ka BP, when modern current systems were established during the middle Holocene sea-level highstand. However, wide-spread muddy deposits in the middle shelf initiated after 3 ka BP which are associated with enhanced sediment fluxes and strengthened winter monsoon. Human activities on recent millennia have play a significant role in influencing the mud deposition on the north-west shelf of the South China Sea, as evidenced by the enrichment of heavy metals in marine sediments.

More extensive land loss expected on coastal deltas due to rivers jumping course during sea-level rise

River deltas are home to hundreds of millions of people worldwide and are in danger of sinking due to anthropogenic sea-level rise, land subsidence, and reduced sediment supply. Land loss is commonly forecast by averaging river sediment supply across the entire delta plain to assess whether deposition can keep pace with sea-level rise. However, land loss and deposition vary across the landscape because rivers periodically jump course, rerouting sediment to distinct subregions called delta lobes. Here, we developed a model to forecast land loss that resolves delta lobes and tested the model against a scaled laboratory experiment. Both the model and the experiment show that rivers build land on the active lobe, but the delta incurs gradual land loss on inactive lobes that are cut off from sediment after the river abandons course. The result is a band of terrain along the coast that is usually drowned but is nonetheless a sink for sediment when the lobe is active, leaving less of the total sediment supply available to maintain persistent dry land. Land loss is expected to be more extensive than predicted by classical delta-plain-averaged models. Estimates for eight large deltas worldwide suggest that roughly half of the riverine sediment supply is delivered to terrain that undergoes long periods of submergence. These results draw the sustainability of deltas further into question and provide a framework to plan engineered diversions at a pace that will mitigate land loss in the face of rising sea levels.

Characterisation of a New Delta‐Zero Natural Mg Solution Alfa‐Mg for Isotope Ratio Measurement and Mg Isotopic Values in Fourteen Matrix Reference Materials

The Mg isotope system helps constrain a variety of geochemical processes in the hydrosphere, lithosphere and biosphere. DSM3 is commonly used as delta‐zero material for relative Mg isotopic measurements. In this study, we have characterised the Mg isotopic composition of a new pure Mg solution (Alfa‐Mg). This solution could potentially be used as a secondary delta‐zero reference material, or as an alternative primary reference material when DSM3 is depleted. The Alfa‐Mg solution has been checked for homogeneity and stability and is available free of charge upon request. The certified δ26/24Mg value relative to DSM3 of the Alfa‐Mg solution is −1.40‰, with an expanded (k = 2) uncertainty of 0.05‰. We also characterised a range of existing Earth‐surface reference materials including soils, riverine sediments and plants. The δ26/24Mg values (relative to DSM‐3) of plant materials range between −1.06 ± 0.03‰ (2s, n = 12) and −0.75 ± 0.03‰ (2s, n = 12). The δ26/24Mg values of soil materials vary from −0.72 ± 0.11‰ (2s, n = 12) to 0.17 ± 0.05‰ (2s, n = 12). The δ26/24Mg values of riverine sediment materials range between −0.66 ± 0.01‰ (2s, n = 12) and −0.17 ± 0.03‰ (2s, n = 12). These data could serve as a reference for inter‐laboratory calibration and quality control, as well as for potential applications in geological and biological fields.

Assessment of microplastics in Irish river sediment

Microplastics (MPs) are environmental pollutants of growing concern, and awareness of MPs pollution in marine and freshwater environments has increased in recent years. However, knowledge of MPs contamination in riverine sediments in Ireland is limited. To address this, we collected and analysed sediment samples from 16 selected sites along the River Barrow. Microplastics were extracted through a density separation method, after which their size, colour, and shape were analysed under a stereo microscope (Optica SZM-2). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to identify polymer types. A total of 690 MPs were recovered from the 16 sites, with fibres as the dominant MP type. The highest concentration of MPs was 155 MP fibres kg−1 wet sediment found in samples collected from Graiguenamanagh, Co. Kilkenny (GK). The majority of the recovered MPs were polyethylene (PE), polypropylene (PP), nylon, and cellulose acetate (CA) fibres. Overall, this study highlighted the presence of MPs in Irish river sediments and provided a baseline for future studies on MPs pollution. Further research is needed to better understand sources, distribution, and effects of MPs in freshwater ecosystems.

Flocculation of Riverine Sediment Draining to the Great Barrier Reef, Implications for Monitoring and Modeling of Sediment Dispersal Across Continental Shelves

AbstractSediment transport models, utilized to guide land management in the Great Barrier Reef (GBR), assume settling velocities for individual silt and clay particles on the order of 0.01 mm/s; however, silts and clays once flocculated exhibit settling velocities on the order of 1 mm/s. In this study, in‐situ (n = 144,912) and laboratory‐dispersed (n = 64) particle size measurements collected using laser diffractometry were compared from nine rivers discharging along 800 km of GBR coastline to determine the extent of in‐situ flocculation. Environmental controls on in‐situ particle size were investigated using decision tree algorithms trained on coeval measurements of salinity, shear rate, and turbidity. Comparison of in‐situ and dispersed particle size measurements demonstrate that suspended‐sediment across all catchments flocculated into larger aggregates with an order‐of‐magnitude difference in median particle size between in‐situ (D50v = 132 μm, σ = 60 μm for all data) and dispersed (D50v = 15 μm, σ = 11 μm for all data) particles. Machine learning algorithms showed excellent promise predicting various measures of in‐situ particle size. Model validation R2 ranged from 0.72 to 0.99, inclusion of catchment as a categorical variable only marginally (<1%) increased R2. These results demonstrate that flocculation is prevalent across all rivers surveyed and that hydrodynamics are more important than inter‐catchment differences (e.g., differences in climate, geology, or land‐use). Implications of widespread flocculation on the determination of end‐of‐catchment sediments loads and subsequent dispersal patterns across continental shelves are discussed to inform the refinement and monitoring of GBR sediment targets.

Combining target analysis with sum parameters-a comprehensive approach to determine sediment contamination with PFAS and further fluorinated substances.

Recent studies aiming at a fluorine mass balance analysis in sediments combined the determination of extractable organic fluorine (EOF) with target analysis. They reported high fractions of unidentified organic fluorine (UOF) compounds, as the target analysis covers only a limited number of per- and polyfluoroalkyl substances (PFAS). For this reason, in this study, a comprehensive approach was used combining target analysis with an extended PFAS spectrum, the EOF and a modified total oxidisable precursor (TOP) assay, which includes trifluoroacetic acid, to determine the PFAS contamination in sediments (n=41) and suspended solids (n=1) from water bodies in Northern Germany (Lower Saxony). PFAS are ubiquitous in the sediments (detected in 83% of the samples). Perfluorinated carboxylic acids (PFCAs) were found in 64% of the samples; perfluorinated sulfonic acids (PFSAs) were detected less frequently (21%), with the highest concentration observed for perfluorooctanesulfonic acid (PFOS). Levels of precursors and substitutes were lower. Applying the TOP assay resulted in an increase in PFCAs in 43% of the samples analysed. In most cases, target analysis and the TOP assay could not account for the EOF concentrations measured. However, as the fraction of UOF decreased significantly, the application of the TOP assay in fluorine mass balance analysis proved to be an important tool in characterising the PFAS contamination of riverine sediments.

Occurrence and distribution of organic corrosion inhibitors (OCIs) in riverine sediments from the Pearl River Delta, South China.

Although soluble organic corrosion inhibitors (OCIs) have been observed globally in surface water, data on their exposures in sediments are still scarce. In this study, a comprehensive investigation on spatial variations and potential sources of OCIs were conducted in riverine sediments from the Pearl River Delta (PRD), one of the most developed and urbanized areas in China. Of 12 OCIs, 7 were detected with the total concentrations ranging from 81.8 to 401.2 ng/g. When the results were compared with those of the water phase, OCIs in the riverine sediments exhibited relatively low concentrations, which was likely due to their low Kow, and they were not expected to be adsorbed onto sediments. The spatial variation of OCIs suggested that the discharge of sewage treatment plants (STPs) effluent could be a major source of OCIs in the PRD region. The total concentrations of OCIs had a significant positive correlation with total organic carbon (TOC) contents, suggesting that they have similar sources. This study strongly indicated that the high consumption of OCIs have led to their wide exposure in different environments in the PRD region and additional ecotoxicological data are needed to evaluate their potential risks in riverine sediments in the future.

Chemical characterization of riverine sediments affected by wastewater treatment plant effluent discharge

This study was aimed at assessing the contribution of wastewater treatment effluents to the contamination profile of the sediments of receiving waterways. Three wastewater treatment plants (WWTP) were addressed, encompassing different population equivalent sizes, urbanization degrees and treatment methods translating differences in expected contamination patterns. Within each WWTP system, the assessment targeted the effluent and sediment samples collected upstream and downstream the effluent discharge point; contaminants belonging to several concerning chemical classes (metals and metalloids; pesticides; pharmaceuticals and personal care products, PPCPs; and polycyclic aromatic hydrocarbons, PAHs) were quantified both in effluent and sediment samples. Clear associations between contaminants present in the effluent and corresponding sediment samples were not always verified. In fact, a noticeable difference between the number or abundance of contaminants detected in effluents and in sediments, suggesting that effluents are not always the most likely source (e.g. PAHs). However, sediment contaminants that were likely sourced by the effluents were also identified (e.g. PPCPs). Sediment analysis offers an important historical view of contamination, especially in flowing recipient ecosystems where any characterization over the water matrix is ephemeral and linking exclusively to the moment of sampling. Hence, sediments should be considered for the establishment of WWTP operational benchmarks regulating the emission of contaminants, which is currently focused mostly on effluent composition thus potentially over/underestimating the longer-term impact of effluent discharge in the recipient waterways.

Managing Fenton-treated sediment with biochar and sheep manure compost: Effects on the evolutionary characteristics of bacterial community

Fenton oxidation is a widely used method for the fast and efficient treatment of contaminated sediment, but few studies have investigated the management of Fenton-treated sediment for resource utilization. In this study, the evolutionary characteristics of bacterial community composition in Fenton-treated riverine sediment were investigated using 16S rRNA gene sequencing after the incorporation of rice straw biochar and sheep manure compost. The Fenton treatment caused a decline in the relative abundance of Bacteroidetes from 39% to 8% on the 7th day, and using biochar and compost rapidly increased the relative abundance of Firmicutes from 13% to 61% and 57%, respectively. Applying 1.25 wt% biochar after the Fenton treatment contributed to high Shannon diversity indices of 4.80, 4.69, and 4.76 on the 7th, 28th, and 56th day, respectively. The reduced differences of Shannon indexes on the 56th day indicated that the bacterial diversity among different treatments tended to be similar over time. The genera Flavisolibacter and Bacillus were representatively detected on the 7th day in the untreated sediment and Fenton/biochar-treated sediment, respectively. The number of feature bacteria decreased significantly from 88 on the 7th day to 29 on the 56th day. The community functions for the carbon, nitrogen, and sulfur cycles were sensitive to the Fenton-treatment and the subsequent treatment with biochar and compost. This study may provide a useful reference for follow-up work on the remediation of contaminated sediment using advanced oxidation processes, and promote the development of resource utilization of amended sediment.

Clay Mineralogical Records in the North Bohai Coast of China in the Last Century: Sediment Provenance and Morphological Implications

Provenance identification of the clastic sediments is critical for paleoenvironmental and morphological reconstruction on the estuarine coast. The North Bohai Coast (NBC) receives tremendous sediments mainly from the Liaohe and Daling fluvial systems. However, the detailed morphological evolution is still unclear due to the lack of precise discrimination of the sediment provenance. In this study, the clay mineralogy of the coastal sediments in the NBC was investigated to determine variation of the fine-grained sediment sources in the last century. The clay mineral content in the modern riverine sediments in the NBC was used to model sedimentary contribution from different fluvial systems. 210Pb and 137Cs radioactivity was measured to constrain the timing of centennial marine–terrestrial sedimentation. The migration of coastline and river channels in the NBC during the last hundred years was reconstructed by digitalizing the historical charts and satellite image. 210Pb following the CIC model and 137Cs dating are considered not applicable for the dated cores, whilst 210Pb using the CRS model yields acceptable chronologic results. Non-linear modeling results of the clay mineral composition are consistent with the lithological and coastal geomorphologic constraints. The outcome of provenance discrimination indicates that clay minerals in western and eastern NBC are mainly derived from the Daling and Liaohe rivers, respectively. In the middle NBC, the Daling River input was relatively major during the 1910s, 1940s, 1980s, and 2000s. The increase of Daling River input in the first two periods resulted from progradation and river channel migration, respectively. The fluvial contribution from the Liaohe River was major in the middle NBC during 1920–1940 and 1950–1970 AD, which attributes to rapid progradation of the Liaohe River and channel migration of the Daling River, respectively.

Trends in surface elevation and accretion in a retrograding delta in coastal Mississippi, USA from 2012–2016

The Grand Bay estuary is in the north-central Gulf of Mexico and lacks riverine sediment input for marsh elevation maintenance. This study quantified trends in surface elevation change and accretion along an elevation gradient within the estuary. Elevation change rates were compared to short (13.71 mm/yr; 95% CI: -2.38–29.81), medium (6.97 mm/yr; 95% CI: 3.31–10.64), and long-range (3.50 mm/yr; 95% CI: 2.88–4.11) water level rise (WLR) rates for the region. Elevation change rates ranged from 0.54 mm/yr (95% CI: -0.63–1.72) to 5.45 mm/yr (95% CI: 4.27–6.62) and accretion rates ranged from 0.82 mm/yr (95% CI: -0.16–1.80) to 3.89 mm/yr (95% CI: 2.90–4.89) among marsh zones. Only the elevation change rate at a Juncus roemerianus marsh located high in the tidal frame was lower than long- (P < 0.001) and medium-range WLR rates (P < 0.01). The elevation change rate at a lower elevation J. roemerianus marsh was higher than the long-range WLR rate (P < 0.05). No marsh zones had elevation change rates that were significantly different from short-range WLR. These results suggest that J. roemerianus marshes higher in the tidal frame are the most vulnerable to increases in sea level. Lower elevation marshes had higher rates of elevation change driven by sediment accretion and biogenic inputs. Other local research suggests that shoreline erosion is a threat to marsh persistence but provides elevation capital to interior marshes. Marsh migration is a potential solution for marsh persistence in this relatively undeveloped area of the Gulf Coast.

Boosting riverine sediment by artificial flood in the Yellow River and the implication for delta restoration

Due to accelerated river-derived sediment deficits, global deltas are presently under an increasing risk of being submerged. To address the fundamental need to save Earth's deltas, boosting riverine sediment supply can be considered a reasonable option, but only if the new supply of sediment is prolonged and greater than the opposing forces: sea level rise, delta subsidence and compaction, and ocean energy. In July 2018, an artificial flood with a peak discharge of 3780 m3/s was released from the Xiaolangdi Reservoir, the last reservoir located on the mainstream of Yellow River, China. From July 03 to July 29, 412 million tons (Mt) of sediment was discharged out of the dam, including 240 Mt. that was previously sequestered within the reservoir. During this event, 140 Mt. of the escaped sediment directly reached the estuary. A total of 245 Mt. of sediment was deposited along the lower river channel bed, of which 83.6 Mt. was eroded once again and transported to the sea in the subsequent five months. This dam-released pulse, coupled with channel diversion and tidal shear fronts, nourished the deltaic area. New land was built, with the submarine offshore gaining up to 2.6 m in seafloor shoaling. Sediment impounded behind the dam, if properly released, could thus become a potential savior for sediment-starved deltas.

Human Intervention–Induced Changes in the Characteristics of the Turbidity Maximum Zone and Associated Mouth Bars in the Yangtze Estuary

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated.

Holocene filling of a narrow estuary in a regressive coast: The Paleo-Kinu Bay region, central Japan

Estuaries are typical depositional systems of transgressive coasts; however, depositional systems with similar geometry also occur on regressive coasts. Such “regressive estuaries” are found in areas of Asia, South America, and Australia that have experienced relative sea-level lowering since the middle Holocene. Regressive estuaries form in incised valleys that were drowned by the last deglacial sea-level rise and receive relatively little riverine sediment discharge. The largest estuary in Japan, with a length of 120 km and a width of 10 km, is in the Paleo-Kinu Bay region. A partly unfilled wave-dominated estuary developed in this region prior to the artificial diversion of the Tone River during the 17th century. In this study, we document the Holocene filling of the Paleo-Kinu Bay on the basis of 14 sediment cores and 171 radiocarbon dates. Unfilled wave-dominated estuary at 7–4 ka has more pronounced wave-built morphologic elements than at 4–0 ka. As a result of sea-level lowering and weakening of tidal currents at 4–0 ka, the bay mouth became narrowed by lateral accretion of sandy tidal river sediments and the central basin was filled by progradation of muddy bayhead delta sediments. The acceleration of the bay mouth narrowing and central basin infilling similar to what happened during 4–0 ka in the Paleo-Kinu Bay region have not been identified in transgressive estuaries, but are common features of regressive estuaries.