Sediment Supply Research Articles

Sediment fluxes within salt marsh tidal creek systems in the Yangtze Estuary

Creeks are essential for salt marshes by conveying water and sediment through this geomorphic system. In this paper, we investigate the mechanisms that determine the residual sediment flux using measurements conducted in tidal creeks in salt marshes of the Yangtze Estuary. A main creek and a secondary creek were studied to explore whether the mechanisms determining residual sediment fluxes through the main creek differ from those in the secondary creek. Measurements in creeks were carried out over 5 years, spanning different months. Sediment import was found during most tides, both in the main creek and the secondary creek, implying that creeks in Chongming generally function as a conveyor belt of sediment into the marsh. However, sediment export can occur during certain overbank tides. When comparing the role of creeks in drainage and sediment delivery, the main creek functions more in delivering sediment while the secondary creek primarily serves as a drainage conduit. To better understand the mechanisms behind sediment fluxes, the residual sediment flux was compared with the residual discharge and the sediment differential (differences in sediment concentration between flood and ebb). Overbank tides generally lead to a net outward discharge as more water from saltmarshes can be concentrated into the marsh creek during ebb tides. This net outward discharge tends to export more sediment during ebb tides. However, due to the sediment abundance during the flood phase in the turbid environment, sediment import can be expected even with the residual export of water. Export of sediment was only found for the few tides with a net outward discharge and a small positive sediment concentration differential. Large negative sediment differentials (larger averaged suspended sediment concentration during ebb tides) have not been observed because the sediment supply during ebb is limited. This paper unravels how the sediment differential and residual discharge contribute to the residual sediment flux, providing a better understanding of sediment dynamics in marsh creek systems.

Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 2. Controls on the Formation of Alluvial Patches

AbstractUnderstanding the development and spatial distribution of alluvial patches in mixed bedrock‐alluvial rivers is necessary to predict the mechanisms of the interactions between sediment transport, alluvial cover, and bedrock erosion. This study aims to analyze bedrock alluviation patterns using a 2D morphodynamic model, and to use the model results to better understand the mechanisms responsible for alluvial patterns observed experimentally. A series of simulations are conducted to explore how alluvial patterns in mixed bedrock‐alluvial channels form and evolve for different channel slopes and antecedent sediment layer thicknesses. In initially bare bedrock low‐slope channels, sediment cover increases with increasing sediment supply because areas of Froude‐subcritical flow enable sediment deposition, while in steep‐slope channels the flow remains fully supercritical and the model predicts so‐called runaway alluviation. For channels initially covered with sediment, the model predicts a slope‐dependent sediment supply threshold above which sediment cover increases with increasing sediment supply, and below which the bedrock becomes fully exposed. For a given sediment supply, the fraction of bedrock exposure and average alluvial thickness converge toward the equilibrium value regardless of the initial cover thickness as long as it exceeds a minimum threshold. Steep channels are able to maintain a continuous strip of sediment under sub‐capacity sediment supply conditions by achieving a balance between increased form drag as bedforms develop and reduced surface roughness as the portion of alluvial cover decreases. In lower‐slope channels, alluvial patches are distributed sporadically in regions of the subcritical flow.

Sensitivity of modelled passive margin stratigraphy to variations in sea level, sediment supply and subsidence

AbstractWe produced a 10 Myr synthetic stratigraphic section using a forward stratigraphic model that generates marine deltaic stratigraphy over geological timescales. We recursively fit the model using a Bayesian inversion algorithm to test: (1) if it could be accurately reconstructed; (2) if the parameters used to create it could be recovered; and (3) the sensitivity of the model output to given model parameters and the attendant physical processes. The original synthetic stratigraphic section was produced with cyclical sea‐level variations of 40 and 30 m with 2.4 and 10 Myr periods respectively. Sediment was also supplied cyclically, in 2.4 and 10 Myr cycles with amplitudes of 30 and 80 tons/100 kyr, respectively, varying from a mean of 232 tons/100 kyr. Parameter values were sampled to fit the model using a Markov chain Monte Carlo algorithm, resulting in a ±5 m (1σ) variation between the experimental output and the original. Sea level varied by ±7 m (1σ) within the posterior distribution of parameters. As a result, both the 10 Myr and 2.4 Myr sea‐level cycles could be extracted from the original output. The variation in sediment supply was approximately ±38 tons/100 kyr (1σ) and, as a result, only the larger long‐term supply variations could be accurately recovered in refitting the model. The variation in thermal, flexural and total subsidence across those parameter sets is less than ±10 m (1σ). The original section experienced 150 m of total subsidence at the depocentre. Our results demonstrate the distinct and interpretable imprint of sea level and subsidence on continental margin stratigraphy can be quantified. Moreover, we conclude that sea‐level change produces a defined effect on the geometries of stratigraphic architecture, and that techniques applied for the purpose of delineating sea‐level variation from continental margin strata have a well‐founded conceptual basis.

Uncertainties of Annual Suspended Sediment Transport Estimates Driven by Temporal Variability

AbstractThe majority of sediment transported from rivers to the global oceans is moved in suspension as fine particles. Thereby, the transported sediment shapes the physical environment regarding erosive and accumulative processes. Temporal variations in sediment supply and transport lead to unquantified uncertainties in annual load estimates, requiring high‐resolution data sets and a sound understanding of site‐specific catchment characteristics. We investigate the temporal variability of suspended sediment transport in four catchments in Germany with highly different discharge regimes and catchment sizes (<1,000 km2 to >100,000 km2). The data set consists of high‐resolution 15‐min turbidity measurements with daily discharge and frequent manual sampling. Utilizing a bootstrap approach based on the 15‐min time series, we assessed the impact of the sampling interval on annual load estimates for less frequent data sets. We use the sediment load exceedance time (Ts80%) as a measure of variability and relate it to uncertainties in annual load estimates. Since low‐frequency data sets rely on sediment rating curves, we performed a sensitivity analysis of the rating parameters a, b, and ε. Our results indicate a negative exponential relationship between Ts80% and uncertainties in annual load estimates. Based on the Ts80%, we can derive the shortest sampling frequency necessary to obtain annual load estimates with an error of <20% over varying discharge regimes. Additionally, Ts80% is linked to rating exponent b, with low b‐values indicating high Ts80%‐values and lower variability, and high b‐values indicating higher variability.

The Signature of Climate in Fluvial Suspended Sediment Records

AbstractArid regions are often characterized by exceptionally high rates of fluvial sediment transport, but the processes responsible for this apparent connection between climate and sediment transport remain unclear. We examined decades of continuous flow records and suspended sediment concentrations from 71 rivers across the United States by comparing the suspended sediment rating curve behavior, quantified using power law coefficients and exponents, to an aridity index. Results indicate that higher aridity correlates with both greater overall suspended sediment concentration and lower sensitivity of concentration to changes in discharge, demonstrating that rivers in arid locations on average have greater suspended sediment transport efficiency across most discharges, and achieve high transport rates at a relatively lower discharge than rivers in temperate climates. Furthermore, based on additional analyses of the Normalized Difference Vegetation Index, specific suspended sediment yield, and a hydrograph flashiness index, we attribute the relationships between sediment transport and climate primarily to differences in vegetation density, precipitation, and runoff, variables that all influence both sediment supply and riverbed grain sorting. Finally, we note that the observed contrasts in sediment transport behavior likely represent climate‐driven differences in the magnitude and frequency of sediment supply rather than annual suspended sediment load, which does not depend significantly on climate. This study highlights a critical connection between multiple interrelated climatic factors and sediment transport, an important finding for future hazard mitigation in a changing climate with rapidly shifting vegetation patterns and hydrology.

Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 1. Numerical Model

AbstractMixed bedrock‐alluvial rivers can exhibit partial alluvial cover, which plays an important role in controlling bedrock erosion rates and landscape evolution. However, numerical morphodynamic models are generally unable to predict quasi‐steady persistent patches of depositional alluvial features under conditions where the sediment supply is less than the sediment transport capacity of the channel. Hence, we present a new two‐dimensional depth‐averaged morphodynamic model that can be applied to both fully alluvial and mixed bedrock‐alluvial channels, and we use the model to gain insight into the mechanisms responsible for the development of sediment patches and patterns of bedrock alluviation. The model computes hydrodynamics, sediment transport, and bed evolution using a roughness partitioning that accounts for differential roughness of sediment and bedrock, roughness due to sediment transport, and form drag. The model successfully replicates observations of bar development and migration from a fully alluvial flume experiment, and it models persistent sediment patches observed in a mixed bedrock‐alluvial flume experiment. Numerical experiments in which the form drag, sediment transport roughness, and bedform stress correction were neglected did not successfully reproduce the observed persistent sediment cover in the mixed bedrock‐alluvial case, suggesting that accounting for these different roughness components is critical to successfully model sediment dynamics in bedrock channels.

EFFECT OF THE AMOUNT OF SEDIMENT SUPPLY ON THE UPSTREAM PROGRESS OF WATERFALLS

EFFECT OF THE AMOUNT OF SEDIMENT SUPPLY ON THE UPSTREAM PROGRESS OF WATERFALLS

Bedforms and sediment dynamics under reduced sediment input and estuarine engineering: the case of the Bou Regreg estuary, Morocco

The Bou Regreg riverine estuary constitutes a microtidal environment located within an urban region and characterized by limited sediment inflow from its catch basin hinterlands. The construction of the Sidi Mohamed Ben Abdellah (SMBA) dam has notably diminished sediment supply, especially coarser fractions. In light of these conditions, this study aimed to classify sediment types according to the Folk and Ward classification, stratify morphometric parameters and temporal fluctuations based on dune types, and ultimately evaluate the sediment dynamics driving observed bedform variations. Using an integrated approach, data collection involved various techniques, including CTD profiling, bottom videography, side-scan sonar imaging, sub-bottom profiling, and Van Veen bottom grab sampling. The bed morphology includes segments of smooth bed, erosional and dredged bottom, localized scour features, and submarine dune fields, displaying notable heterogeneity. Acoustic seabed reflections indicated two bedform clusters with maximum lengths of 42.6 m and heights of 1 m. Flow velocities were linked to the presence of 2D and 3D dune forms. Morphological variations encompassed straight, sinuous, and barchan dune configurations, mainly asymmetric with NNW–SSE seaward orientation. Sediment mean size fluctuations spanned from muddy to medium sand deposition. The findings revealed the intricate interplay of dynamics shaping dune field configurations and the heterogeneous dispersion of bedforms throughout the estuary likely due to a sharp decline of sediment discharge caused by the dam upstream and coastal engineering. The study emphasizes the dynamic nature of dunes, highlighting their sensitivity to shifting hydrodynamic conditions and sediment availability, leading to morphological fluctuations over time.

Provenances of Upper Quaternary sediments in the Northern Caspian Sea

This paper presents a generalization of the clay minerals composition in the sediments of the KOR-3 core located in the central part of the Northern Caspian Sea (19 samples). The history of changes in the different river basins contribution to the sedimentary supply in the central part of the Northern Caspian Sea could be traced by changes in clay mineral ratios in the Upper Quaternary sediments. Changes in the ratio reflect differences in material sources during the first and second stages of the Khvalynian transgression of the Caspian Sea — one of the most controversial stages in the history of the Caspian Sea. Consequently, our conclusion is that there were different causes of sea-level uprise at different stages of the transgression. The subsequent events (deep Mangyshlak regression and New Caspian transgression) are not associated with abrupt changes in the inflow of rivers into the Northern Caspian Sea, according to the composition of clay minerals.

A Comparative Reservoir Study of Zubair and Nahr Umr Formations in Subba Oilfield, Southern Iraq

The lower Cretaceous sandstones of Zubair and Nahr Umr formations are the main producing reservoirs in Subba oilfield in southern Iraq. Key differences in their petrophysical and depositional attributes exist affecting their reservoir characteristics. The evaluation of well logs and core porosity-permeability data show better reservoir properties in Nahr Formation. The Litho-saturation logs indicate greater thickness of oil-saturated reservoir units for Nahr Unr Formation associated with lower values of shale volume, and higher values of effective porosity. In addition, higher values of permeability for Nahr Umr Formation is suggested by applying porosity-irreducible water saturation cross plot. The reducing reservoir quality of Zubair Formation sandstones is related to finer grains of sandstone reservoirs, higher clay volume, and more effect of compaction resulting from greater depth. A reservoir layering scheme is proposed by sequence stratigraphic analysis leading to the identification of systems tracts and their key surfaces within Zubair and Nahr Umr formations, and forms the basis for predicting of reservoir architecture and quality. Laterally continuous, and thick amalgamated sandstone reservoirs are formed during highstand systems tract in Nahr Umr Formation as a result of low accommodation/sediment supply conditions. Within the transgressive systems tract of Zubair Formation, the sandstone reservoir bodies become isolated as accommodation exceeds sediment supply.

Texture and major element geochemistry of channel sediments in the Orsang and Hiren River Basins, Gujarat, India: Implications for provenance and weathering

Size, shape, degree of sorting, and composition of sediments in the river channels are controlled by climate, lithology, weathering, sorting, and medium of transportation. The present investigation is focused on the grain-size and geochemical analysis of the channel sediments of the Orsang and Hiren river basins. Major outcrops in the study area are Archaean granites, granitic gneisses, Upper Cretaceous to lower Eocene Deccan Volcanic Basalts (DVB), Quaternary sediments and minor proportion of Proterozoic low grade metamorphic rocks. The sediments are poorly to moderately sorted, very finely skewed, suggesting its derivation from heterogenous sources, while the kurtosis value indicates a high-energy depositional environment. The sediments are with gravelly sand texture and the mean grain size is varying from 581.9µm to 1284.2µm. The DVB provenance of the Hiren river basin and granitic provenance of the Orsang river basin is clearly reflected in the texture and geochemical composition of sediments. The TiO2 and Fe2O3 contents of sediments from the Hiren river basin are distinctly higher and are comparable to the basalts of the Saurashtra region of the Deccan Province. Sediments collected after Orsang and Hiren rivers confluence and from Narmada river show higher concentration of felsic sources, indicating that Orsang river's sediment supply significantly outweighs Hiren rivers. The arkosic-litharenite nature points towards less transportation and moderate chemical weathering for the Orsang river sediments. The low Chemical Index of Alteration (CIA) values (Avg. 48.45 and 56.99 for Orsang and Hiren rivers, respectively) and A-CN-K plot also suggest the supply of sediments from minimally weathered detritus under a semi-arid condition.

A workflow for turbidite reservoir characterization—a case study of the Macedon member, Northern Carnarvon Basin, NW Australia

Deep-water turbidite systems on passive continental margins are of interest for oil and gas exploration. However, their complexity poses challenges for reservoir characterization. In this study, we proposed a reservoir characterization workflow for the Macedon member turbidite, employing a combination of 90° phase adjustment, geobody extraction, and genetic inversion, based on the abundant well logging and seismic data from the Enfield field, Northern Carnarvon Basin. Our workflow involved seismic sedimentology to determine the morphology of sand bodies and inversion to determine the net reservoir range, resulting in 3D geological attribute modeling. We applied a 90° phase adjustment correlated seismic events and well logging responses. By stratal slice interpretation and geological body extraction, it was revealed the turbidite reservoir distribution. Finally, we achieved net reservoir characterization of the Macedon member through genetic inversion porosity and geostatistical methods. The results showed that the Macedon turbidite reservoir can be divided into the top and base reservoirs. The top reservoir is sheet-like, and the base reservoir is channelized. The average porosity of the former was 24%, while the average porosity of the later is 20%. The top reservoir has better reservoir quality. Furthermore, we discussed sea level changes affect turbidite distribution and reservoir quality. During the Falling Stage Systems Tract (FSST), the long transportation distance led to relatively less sediment supply and a low sand/mud ratio, resulting in confined, channelized, poor quality turbidite reservoir. In contrast, during the Lowstand Systems Tract (LST), unconfined, amalgamated, good quality turbidite sheet reservoirs were formed. The improved workflow based on seismic sedimentology presented in this article proves effective in characterizing complex reservoirs and contributes to the simplified and efficient management of reservoirs.

Depositional dynamics and vegetation succession in self-organizing processes of deltaic marshes

Global deltaic marshes are currently facing a multitude of pressures, including insufficient sediment supply, rising sea levels, and habitat loss. Consequently, unraveling the internal regulatory mechanisms within deltaic marshes is of paramount importance. Here, we harness years of observational data and high-resolution numerical models to uncover depositional dynamics and vegetation succession in self-organizing processes of deltaic marshes. Our findings indicate that the colonization of salt marsh vegetation triggered a robust phase of growth in the initial stages of river deltas formation. However, as vertical accretion intensifies and inundation decreases, the delta is driven towards a state of critical slowing down due to insufficient sediment supply. We have captured a pivotal turning point in the evolution of deltaic marshes. In accordance with the critical submergence threshold we have established, when the inundation time of deltaic marshes exceeds 0.97 h/d, these salt marsh platforms sustain a higher annual growth rate. Conversely, when the inundation time of deltaic marshes falls below 0.97 h/d, the interannual accretion rate continues to decrease. Our research reveals that, in the absence of human disturbances, the deposition rate in deltaic marshes transitions from growth to decline. During this period, the delta undergoes an interesting succession of pioneer salt marshes (Suaeda salsa) and high-elevation salt marshes (Phragmites australis). Even without reductions in sediment input due to human activities, the vertical deposition rate within deltaic marshes will still shift from acceleration to deceleration under the influence of this internal negative feedback regulation. This adaptive capacity of marshes may foreshadow that when observing a slowdown in vertical accretion on deltaic marsh platforms, it cannot be solely attributed to reductions in sediment input caused by human activities.

Do topographic changes tell us about variability in aeolian sediment transport and dune mobility? Analysis of monthly to decadal surface changes in a partially vegetated and biocrust covered dunefield

Vegetation and biological soil crust (biocrust) cover can have a stabilizing effect on dunes by fixing sediment in-place and increasing surface roughness, thus limiting dune mobility, sediment transport, and erosion. These biological effects influence rates of aeolian activity and thus surficial changes, though variability in wind and sediment supply may obscure these topographic effects. In this study, we compare monthly measures of sediment transport and decadal estimates of dune mobility to repeat topographic changes measured as a net volume change in sediment storage (difference in volume between all positive and negative topographic changes) and total volume change (absolute summed volume of all positive and negative changes) for areas of bare, vegetated, and biocrusted sand within a dunefield with limited sediment supply and unimodal winds. We found that monthly net volume changes normalized by area were similar between bare sand and sand with at least 20 % vegetation cover. However, total volume change was significantly greater for bare sand and correlated with monthly sediment flux estimates (R2 = 0.46), though the relationship was significantly improved by including monthly changes in surface roughness (R2 = 0.8). Longer-term decadal trends in topographic change showed larger total volume changes with the greatest decreases in vegetation canopy cover. Additionally, decadal total volume changes strongly correlated with estimates of dune mobility (R2 = 0.99). We also found that increased total volume changes did not necessarily signal increased net volume changes for all land cover types. Specifically, increases in total volume change for bare sand resulted in near equal or lower net volume changes, as both positive (deposition) and negative (erosion) change increased with sediment transport. Conversely, less mobile land covers, such as biocrust covered sand, increased in erosion without significant increases in total volume change, demonstrating that more stable surfaces might exhibit a larger topographic change imbalance than mobile sediment surfaces under the same conditions. This study highlights the importance of considering multiple measures of topographic change for interpreting sediment mobility, transport, and availability. Additionally, we hypothesize a novel framework for remote sensing-based empirical studies aimed at interpreting aeolian landscape evolution resulting from climate change effects on weather as well as biological controls such as vegetation and biocrusts.

A new framework for evaluating dust emission model development using dichotomous satellite observations of dust emission

Dust models are essential for understanding the impact of mineral dust on Earth's systems, human health, and global economies, but dust emission modelling has large uncertainties. Satellite observations of dust emission point sources (DPS) provide a valuable dichotomous inventory of regional dust emissions. We develop a framework for evaluating dust emission model performance using existing DPS data before routine calibration of dust models. To illustrate this framework's utility and arising insights, we evaluated the albedo-based dust emission model (AEM) with its areal (MODIS 500 m) estimates of soil surface wind friction velocity (us∗) and common, poorly constrained grain-scale entrainment threshold (u∗ts) adjusted by a function of soil moisture (H). The AEM simulations are reduced to its frequency of occurrence, P(us∗>u∗tsH). The spatio-temporal variability in observed dust emission frequency is described by the collation of nine existing DPS datasets. Observed dust emission occurs rarely, even in North Africa and the Middle East, where DPS frequency averages 1.8 %, (~7 days y−1), indicating extreme, large wind speed events. The AEM coincided with observed dust emission ~71.4 %, but simulated dust emission ~27.4 % when no dust emission was observed, while dust emission occurrence was over-estimated by up to 2 orders of magnitude. For estimates to match observations, results showed that grain-scale u∗ts needed restricted sediment supply and compatibility with areal us∗. Failure to predict dust emission during observed events, was due to us∗ being too small because reanalysis winds (ERA5-Land) were averaged across 11 km pixels, and inconsistent with us∗ across 0.5 km pixels representing local maxima. Assumed infinite sediment supply caused the AEM to simulate dust emission whenever P(us∗>u∗tsH), producing false positives when wind speeds were large. The dust emission model scales of existing parameterisations need harmonising and a new parameterisation for u∗ts is required to restrict sediment supply over space and time.

Climate and mineral accretion as drivers of mineral‐associated and particulate organic matter accumulation in tidal wetland soils

AbstractTidal wetlands sequester vast amounts of organic carbon (OC) and enhance soil accretion. The conservation and restoration of these ecosystems is becoming increasingly geared toward “blue” carbon sequestration while obtaining additional benefits, such as buffering sea‐level rise and enhancing biodiversity. However, the assessments of blue carbon sequestration focus primarily on bulk SOC inventories and often neglect OC fractions and their drivers; this limits our understanding of the mechanisms controlling OC storage and opportunities to enhance blue carbon sinks. Here, we determined mineral‐associated and particulate organic matter (MAOM and POM, respectively) in 99 surface soils and 40 soil cores collected from Chinese mangrove and saltmarsh habitats across a broad range of climates and accretion rates and showed how previously unrecognized mechanisms of climate and mineral accretion regulated MAOM and POM accumulation in tidal wetlands. MAOM concentrations (8.0 ± 5.7 g C kg−1) (±standard deviation) were significantly higher than POM concentrations (4.2 ± 5.7 g C kg−1) across the different soil depths and habitats. MAOM contributed over 51.6 ± 24.9% and 78.9 ± 19.0% to OC in mangrove and saltmarsh soils, respectively; both exhibited lower autochthonous contributions but higher contributions from terrestrial or marine sources than POM, which was derived primarily from autochthonous sources. Increased input of plant‐derived organic matter along the increased temperature and precipitation gradients significantly enriched the POM concentrations. In contrast, the MAOM concentrations depended on climate, which controlled the mineral reactivity and mineral–OC interactions, and on regional sedimentary processes that could redistribute the reactive minerals. Mineral accretion diluted the POM concentrations and potentially enhanced the MAOM concentrations depending on mineral composition and whether the mineral accretion benefited plant productivity. Therefore, management strategies should comprehensively consider regional climate while regulating sediment supply and mineral abundance with engineering solutions to tap the OC sink potential of tidal wetlands.

Quantitative provenance study of sediments in the coastal tidal flats of central Jiangsu based on grain-size End-Member analysis

The coastal mudflats in central Jiangsu Province are influenced by the sediment supply from the Yangtze River and the abandoned Yellow River. However, the sources of sediment in this area have yet to be confirmed, and quantitative studies have been limited. In this study, we addressed this gap by obtaining sediment core samples with lengths of approximately 100 cm from Dongtai and Sheyang, respectively, on the central coast of Jiangsu Province in 2018. The sediment sources were identified and quantitatively evaluated using a grain-size end-member(EM) model. The findings revealed that (1) Sheyang was decomposed into five EMs, with EMs 1-3 originating from the abandoned Yellow River, EM 4 originating from the Yangtze River and the North Jiangsu radial sand ridges (NJRSR), and EM 5 originating from the NJRSR. Over a century scale, the abandoned Yellow River contributed 73.91% to sediment deposition in Sheyang, while the Yangtze River and the NJRSR contributed 26.09%. (2) Dongtai was decomposed into six EMs, with EMs 1-4 originating from the abandoned Yellow River, EM 5 originating from the Yangtze River, and EM 6 originating from the Yangtze River and the NJRSR. Over a century scale, the abandoned Yellow River contributed 70.55% to sediment deposition in Dongtai, and the Yangtze River and the NJRSR contributed 29.45%. (3) On a temporal scale, the contribution of the abandoned Yellow River to sediment deposition on the central mudflats of Jiangsu Province showed an increasing trend. Spatially, the contribution of the abandoned Yellow River to sediment deposition decreased from north to south. (4) There were significant changes in the sediment sources of Sheyang and Dongtai at the depths of 56 cm and 60 cm, respectively, which both experiencing an increased sediment supply from the abandoned Yellow River. In Dongtai, the sediment on the southern side had increased contributions from the abandoned Yellow River due to a reduction in the sediment supply from the Yangtze River. In Sheyang, the sediment on the northern side had increased contributions from the abandoned Yellow River due to the introduction of Spartina alterniflora, despite no actual changes in the sediment sources.

Geomorphic adjustments of channel bars in the braided reach of the lower Yellow river from 1986 to 2018

Channel bars are an important aspect of braided river morphology, and have a significant effect on flood control, waterway navigation and the aquatic ecological environment. In this study, the number, area and shape index of channel bars in the braided reach of the Lower Yellow River (LYR) were determined based on remote sensing images from 1986 to 2018, and the evolution characteristics of channel bars were investigated temporally and spatially. The effects of flow regime, sediment supply and bank erosion were also investigated as part of geomorphic adjustments of channel bars in the braided reach. The findings of this study indicate that the number and area ratio of channel bars were lower in the pre-dam period when water levels were higher and higher in the post-dam period when water levels were lower. More variance in the peak discharge may lead to more variance in channel bars indices in the braided reach. The number of channel bars per unit width is closely related to the specific stream power in the braided reach, and a larger specific stream power leads to more channel bars. The total area of channel bars increased with an increase in the reach-scale bankfull width, which indicates that bank erosion was closely associated with the adjustments of channel bars in the braided reach of the LYR.

Fluid mud induced by periodic tidal advection and fine-grained sediment settling in the Yangtze estuary

This study presents in-situ observations of fluid mud in the Yangtze Estuary via a bottom tripod. Six occurrences of periodic fluid muds (F1-F6) were observed at high slack water, with suspended sediment concentration (SSC) exceeding 10 kg/m3. The thickness of the fluid mud varied across the six occurrences, reaching a maximum of 0.32m. Notably, temperature and salinity anomalies were observed within the fluid mud. The formation of fluid mud was found to be influenced by turbulence, with turbulence kinetic energy (TKE) below 5×10-4 m2/s2 favoring settling as the primary cause of formation. The critical shear stress (τcw) for fluid mud formation was estimated to be approximately 0.09 Pa. The formation of fluid muds during early tidal cycles was attributed to liquefaction and bed erosion resulting from strong waves. However, in most cases, the wave energy was insufficient for erosion, suggesting that advection played a role in sediment supply during the subsequent tide cycles. The increased cross-channel current velocity during flood facilitated the lateral transport of a significant amount of eroded sediment from shallow water into the channel. Due to its short duration, the fluid mud layer was entrained by the current before consolidation. Enhanced turbulence led to the breakdown of fluid mud at the early ebb. The critical τcw for fluid mud breakdown depended on the density and duration of the fluid mud, with a maximum value of up to 0.70 Pa. The absence of the fluid mud layer during low slack water and neap tide was associated with a reduction in advection and tidal pumping, disrupting the original sediment balance between supply and demand. These data provide valuable insights into the formation and breakdown of fluid mud, contributing to estuarine hydrodynamic modeling studies and enhancing the understanding of estuary dynamics.

Response of a small, forested stream to a large input of sediment

In small headwater channels coupled to hillslopes, episodic supplies of sediment from events like landslides can dominate local channel morphodynamics and impact ecosystem health. In this study we investigate the response of a small mountain stream in British Columbia to a sediment input event that was up to 150 times the volume of sediment transported in an average flood year caused by the upstream installation of a road bridge. Using four years of repeated UAV surveys, bedload tracers, and other field monitoring techniques, we examined the spatial and temporal changes in sediment storage along the reach, identified changes to channel morphology, and characterized the interaction between the channel bed and the supplied sediment. Historical monitoring of the reach allowed for a comparison of the response to nearly 20 years of information about bedload transport and morphologic change. We observed that most of the change within the reach occurred during the first flood after bridge installation. The supplied sediment mostly moved across the bed surface with little interaction, as tracers installed in the bed had lower than expected rates of mobility. A narrow, steep segment of the channel proximal to the bridge installation stored little sediment, whereas a wider, less steep segment further downstream absorbed much of the change. This effect was exacerbated by a logjam that formed during the first flood after bridge installation. Despite the relative magnitude of the sediment transport event, there was little change to the channel pattern, with the added sediment increasing the relief of the already existing morphologic units. This suggests river managers should focus their restoration attention on segments likely to respond to a disturbance and that the pool-riffle channel morphology can persist across a wide range of sediment-supply conditions.