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A one-dimensional numerical model (HEC-RAS) was calibrated to simulate morphological changes in the study reach of the Nile River downstream of Esna barrages by using the available field data of the years 2010 and 2013. The presented research utilizes the synergies between GIS and HEC-RAS in studying the morphological changes in the study area that affects directly the navigation condition and Proposes the optimum navigation path in the selected study reach based on the numerical model results data. Using data measurements, the model was calibrated and verified. It gave a reasonable agreement between the calculated and measured values for the water level and the morphological changes in the selected cross-sections and along the River's thalweg with accuracy 99.63% and 96.66% for hydraulic and sediment calibration and verification. It can be concluded from several runs of the model that the best equation for predicting morphological changes within the study area is the Ackers-White equation. Finally, the results indicate that the HEC-RAS model presents a satisfying performance for sediment transport.

Sediment deposition is a significant feature of a dynamic floodplain river. As the river's carrying capacity decreases, it is forced to deposit sediment on the river bed. However, due to the construction of some hydraulic structures over the river bed at different times (bridges, dams), the velocity of the river in the floodplain area decreases with accelerating sedimentation process, which hinders water transport. The sub-catchment basin (Nabadwip-Kalyani stretch) of the Bhagirathi–Hooghly river in West Bengal (Ghosh et al. 2020), India is frequently used as a means of water transport by local stakeholders. Because of sedimentation, the thalweg depth of the river is constantly decreasing in Nabadwip–Kalyani stretch and the water transport is being disrupted. Therefore, it is very important to know the function of hydraulic structures over the river bathymetry and resultant sediment flow, transportation, and sediment deposition in this region. Therefore, this article mainly assesses to develop a 1D hydro-dynamic sediment transport model on river morphological change through seasonal quasi-unsteady flow accumulation and sediment budget in a given year (2018). The hydro-dynamic model is calibrated by 1D HEC-RAS v 5.0.7 software (Gibson et al. 2017) based on seasonal quasi-unsteady flow using various empirical equations and Manning's roughness coefficient and validated with R2, NSE, and RSR. The model also examines the role of hydrodynamic structures in scouring and sedimentation due to the location of these structures above the river. This model output will help local stakeholders to understand the amount of sediment in the river and keep river transport viable in all seasons.

This study aims to describe how the construction process of a reservoir dam affects and changes in river morphology such as cross-section changes and total sediment movement in different annual and seasonal scales. Cross-sectional and profile changes, and spatial distribution of differences (degradation and aggradation areas) between topographic measurements and bend variations are also discussed. For this purpose, a field measurement campaign was carried out on Buyukkumla stream, where Buyukkumla dam is built, from the spillway of the dam to the Marmara Sea. Topographic measurements were taken every six months from the beginning of the dam’s construction until the body is completed. Precise measurements were made across the entire stream, including the thalweg of the mainstream bed, up to 25 m on the right and left bank at 20-m intervals, and at the intermediate points where the required cross-sectional changes were found. The aggradation and degradation volumes were determined as much as the number of cross-sections available for each region at different time periods. Besides, the reasons for these aggradations and degradation were investigated considering the construction activities and meteorological conditions. Finally, our results show that there is a correlation between dam construction activities and downstream geomorphology changes. It is also observed that excavation activities in dam construction have a significant impact and regulate downstream and the changes in annual and semi-annual periods differ.

Deposits in dam areas of large reservoirs, which are commonly composed of fine-grained sediment, are important for reservoir operation. Since the impoundment of the Three Gorges Reservoir (TGR), the sedimentation pattern in the dam area has been unexpected. An integrated dynamic model for fine-grained sediment, which consists of both sediment transport with water flow and gravity-driven fluid mud at the bottom, was proposed. The incipient motion driven by gravity in the form of fluid mud was determined by the critical slope. Shallow flow equations were simplified to simulate the gravity-driven mass transport. The gravity-driven flow model was combined with a 3D Reynolds-averaged water flow and sediment transport model. Solution routines were developed for both models, which were then used to simulate the integral movement of the fine-grained sediment. The simulated sedimentation pattern agreed well with observations in the dam area of the TGR. Most of the deposits were found at the bottom of the main channel, whereas only a few deposits remained on the bank slopes. Due to the gravity-driven flow of fluid mud, the deposits that gathered in the deep channel formed a nearly horizontal surface. By considering the gravity-driven flow, the averaged error of deposition thickness along the thalweg decreased from −13.9 to 2.2 m. This study improved our understanding of the mechanisms of fine-grained sediment transport in large reservoirs and can be used to optimize dam operations.

Channel junctions and lateral intakes are always caused turbulence in the passing flow. In this research, an experimental study was carried out to explore a possible method for sediment control for a series of skimming walls and a single spur dike, simultaneously. In order to direct the flow into the diversion channel and increase the skimming wall function, a single spur dike was utilized at the opposite side of the intake channel. The flow patterns for three conditions, including (1) without structures, (2) with skimming wall and (3) skimming wall and spur dike, are considered. The results showed that the use of skimming wall and a combination of skimming wall and spur dike makes it possible to direct the thalweg toward the intake port. In addition, by using the skimming wall, a trench is made toward the intake, which in turn increases the impoundment efficiency by 81% in the skimming wall combined with spur dike and up to about 66% for using the skimming wall.

Mouth bars are important morphological units in deltaic and estuarine systems, which have encountered with significant topography variations driven by anthropogenic interferences and natural forcings. On the basis of seven selected maritime charts covering from 1960s to 2016, we focus on the morphological evolution of mouth bar with bifurcating channels in the Modaomen Estuary, the main fluvially-dominated estuary of the Pearl River Delta. The bar area is relatively in equilibrium with barely aggregation and degradation before 1990 while overall erosion was found thereafter. Sediment volume of the mouth bar between the −2 m and −7 m isobaths drastically decreased after 1990s. Correspondingly, the area of −4 m isobaths of the bar stepwise decreased from about 18 × 106 m2 to 2 × 106 m2, indicating the body of mouth bar above −4 m almost disappeared. The two side channels also suffered significantly morphological evolution. The West Channel (WC) was subjected to uninterrupted erosion. Especially during 2000–2005, the WC below the −2 m isobath experienced remarkable erosion with the increment of water volume reaching 6.7 × 106 m3. The East Channel (EC) had the potential to expand seaward between 2000 and 2011 yet encountered with accumulation after 2011. Meanwhile, the thalweg of the EC seemed hard to distinguish and the WC displayed more meandering thalweg. Furthermore, Empirical Orthogonal Function was applied to quantify the bathymetric changes with independent modes. The temporal variations of fluvial sediment input displayed significant decreasing trend at MK from 1960 to 2014, which presented a statistically significant relationship with the third mode. However, it only accounts for 13.4% of the overall evolution patterns. Although the remarkable reduction of sediment input generally associates with the degradation in bar area, the sediment load is probably not the primary mechanisms driving morphological changes in the Modaomen Estuary. Anthropogenic interferences including large-scale reclamation and intensive sand excavation should be responsible for the morphological changes.

There are about 66 tributaries distributed in the Three Gorges Reservoir (TGR) area. As obvious deposition has been observed at estuary of most tributaries estuary since the water-impoundment of TGR, the possibility of sedimentation leading to form entrance bar at estuary of tributaries should be highly concerned. Based on data of topography and cross-sections, the DEM digital elevation model is established to calculate the aggradation volume at the estuary of tributaries in TGR, also the scouring and silting thickness at estuary will be demonstrated. Changes of cross-sections and thalweg caused by aggradation are also studied. The results show that: (1) The aggradation volume at estuary of 66 tributaries in TGR was 180 million m3 from March 2003 to November 2011; (2) hydrodynamic conditions at estuary of tributaries weakened after the operation of TGR, which is the root cause of sediment deposition. The extent of sediment silting was affected by sediment from the tributaries and backward water from the mainstream, meanwhile the dynamic state of river course at estuary had an important influence on the range of aggradation; (3) sediment deposited at the estuary mainly derived from backward water of mainstream, and less from the tributaries. As sediment concentrations both in mainstream and tributaries decreased obviously and will be continually maintain a low level, there is little risk for the formation of entrance bar of tributaries in TGR.

Metal(loid)s in the reservoir sediment tend to be released into the water column when encountering disturbances and thus pose threats to the aquatic system. In this study, sediment and pore water samples collected from eight cross sections in the Biliu River Reservoir (Dalian, China) were analyzed to determine the spatial distributions of six metal(loid)s and their associations with reservoir morphometry and hydrodynamics. The results show that total metal concentrations of the sediments are higher at the sites with greater water depths and are influenced by the reservoir morphometry. Mn is of great concern with respect to its increasing total concentration from the upstream sites to the dam sites. According to the improved BCR sequential extraction procedure, the acid-soluble fraction of Mn increases along the thalweg to the dam, implying the soluble Mn2+ in the upstream hypolimnion, and sediment is possible to be transported longitudinally by water currents. For Fe, Mn, Pb, Cu, and Zn, the reducible fraction accounts for more than 15% of the total metal concentration, which suggests that Fe–Mn (hydr)oxides could be important in scavenging these metals. High Mn concentrations in pore waters close to the dam, with an average value of more than 40 mg/L, give rise to significant Mn diffusive flux up to 296.1 mg/m2/day.

The 2011 flood, an approximately 500-yr flood on the Missouri River, built many large sandbars that have since been modified by eolian and fluvial processes. The ‘59-mile’ segment of the Missouri National Recreational River, located between Yankton, South Dakota and Ponca, Nebraska, USA is a relatively free-flowing stretch of the river downstream of a major dam which has drastically reduced sediment loads since 1955. Critical sandbar habitat has gradually declined on this stretch of the river, presenting challenges to river management. Satellite and LiDAR imagery was used to measure changes in area, volume, and surface characteristics of sandbars over three years since the 2011 flood. The Portable in situ Wind Erosion Lab (PI-SWERL) was used to measure the potential for bare sandbars to emit dust. Our results suggest that eolian processes are important in the early evolution of sandbars, as the wind modifies the surface into dunes and gravel lags. Furthermore, sandbars are important local sources of dust until gravel lags stabilize the surface, limiting further eolian deflation. The lowering of sandbar surfaces by wind erosion makes them more susceptible to being overtopped by high water events. Fluvial erosion along the margins of the sandbars ultimately reduces their area over time. Vegetation growth further stabilizes sandbar surfaces. The rate of eolian and fluvial erosion ultimately depends on the position of the sandbars relative to the thalweg as well as wind direction. New sandbars are rarely built by fluvial processes on a regulated river like the Missouri, and are in a constant state of erosion and modification, ultimately limiting their longevity as quality habitat.

Sediment supply and sea level interact to control sediment flux to deep-water submarine fans. Although some fans continue to be active during times of rising sea level, the source of sediment is not always clear and may be dominated by reworking in high energy coastal areas rather than reflecting erosional signals directly from the source drainage basin. We present new age and geochemical provenance data from cores covering the last ~20 ka that show continuous deep-water sedimentation through the Indus submarine canyon since at least ~11 ka, despite the cessation of sedimentation on the upper fan around that time. Large turbidity flows mantled terraces >200 m above the thalweg throughout the Holocene and their deposits show trends in grain size and geochemistry that we interpret to record direct supply from the river mouth and little reworking of older deposits eroded by longshore currents, storm waves, slumping, or sliding, at least at 4.7–9.0 ka. We use Nd–Sr isotope compositions to show that sediments within the canyon and in the shelf clinoform to the east of the canyon are similar to the Holocene river mouth, suggesting direct supply from the Indus River to the eastern clinoform and into the canyon. The sediment storage time on the shelf before redeposition would be no more than ~8 k.y., and likely much less during the Early-Mid Holocene (4.7–9.0 ka). Sr–Nd isotopes also indicate that significant reworking of sediment older than 8 ka during sea level rise is excluded. Thus, coherent erosional pulses (signals) in the river, likely caused by climatic disturbances, are communicated to the canyon at least since ~9.0 ka, suggesting that sediment supply, modulated by climate in the Indus basin, dominates over sea level in controlling canyon sedimentation in high sediment supply settings, although the vast majority of the sediment supply is stored on the shelf and in the delta. Our study for the first time allows the differentiation between reworking within the canyon and direct supply from the river in a major submarine canyon.

The objective of this study was to evaluate the input of suspended sediment from the slope toe in the São Francisco River channel, in Northeast Brazil, under different soil bioengineering techniques. Sediments were collected in the years of 2013, 2014 and 2015, in five transects crossing the river channel. Sediment input in the sampling points was higher in the year of 2013, i.e., the year of the highest river discharge. Sediment supply to the river channel for a period of 3 years (2013, 2014 and 2015), was evaluated at 20, 40 and 60% depth, along five different transects (P1= riverbank, P2= beginning of the thalweg, P3= middle of the thalweg, P4= end of the thalweg, and P5 = margin of the side sandybar), oriented by the presence or absence of erosion control techniques (treatments) such as: 1-Natural Vegetated Slope; 2-Vegetated Riprap; 3-Eroded Slope; 4-Live Cribwall and 5-Vetiver grass Contour Line. Sediments input was different in all evaluated transects, and the one identified as Eroded Slope at 20% depth presented the lowest amount of suspended sediment load. There was a decrease in the total amount of suspended sediment in the evaluated periods, probably due to the progressive decrease in the river discharge, and the protection provided by the soil bioengineering techniques.

International audience; Over the last two centuries, rivers have been strongly regulated, inducing notable sediment starvation and flow reduction. This regulation has altered aquatic habitats, biocenosis and ecosystem services. Gravel augmentation (GA) is increasingly promoted to restore bedload transport, rejuvenate bed sediments and diversify aquatic and riverine habitats. However, practical questions remain in terms of efficiency and sustainability. The objective of this study is to provide unprecedented feedback from three GA experiments conducted along the Rhine River downstream of the Kembs dam (France/Germany). The restored sites were monitored over periods of 1 to 7 years and the stockpile deposits (SDs) and channel responses were compared to highlight the strengths and limitations of such experiments. The monitoring was based on topo-bathymetry, bedload tracking and grain size surveys. Two-dimensional hydraulic modelling was also performed to link the SD erosion patterns to the critical flow conditions. We show that (i) SD erosion is dependent on the grain size of the introduced sediments and the local flow velocities, (ii) sediment dispersion occurred mostly along the thalweg and increased over time, with a progressive decrease in the virtual velocities, (iii) bathymetric simplification and bed grain size fining were observed when downstream sediment diffusion began, (iv) in the mid-term (~5 years), local channel diversification occurred due to the fragmentation of sediment waves, with deposition occurring preferentially on riffles, and (v) new sediment starvation conditions appeared when the sediment waves travelled downstream in the absence of an upstream sediment supply. These results allow us to propose operational recommendations and management scenarios to improve both the efficiency and the sustainability of GAs conducted in large regulated rivers.

Sediment transport, turbidity, and dissolved oxygen were evaluated during six consecutive water years (2013–2018) of drawdowns of a flood control reservoir in the upper Willamette Valley, Oregon, USA. The drawdowns were conducted to allow volitional passage of endangered juvenile chinook salmon through the dam's regulating outlets by lowering the reservoir elevation to a point where the historical streambed was exposed and transported water and sediment through the reservoir dam. Sediment loads during the drawdown were highest in the first year of monitoring, with a computed value of 40,200 metric tons over a 5-day drawdown, followed by 5 years of lower sediment loads and lower sediment transport rates, suggesting that much of the stored sediment within the reservoir thalweg was transported out of the reservoir in the early years of the consecutive drawdowns. Suspended sediment concentrations (SSC) computed using turbidity and streamflow data resulted in maximum SSC at the onset of the drawdowns, with the highest computed values occurring during the water year 2017 drawdown at 17,500 mg/L (turbidity = 2,990 FNU), and average drawdown SSC values ranging from 654 to 3,950 mg/L for the six years of monitoring. Computed SSC were on the lower range of concentrations that could be harmful to out-migrating juvenile salmon published in other studies. High amounts of particulate organic matter and sand-sized material in drawdown SSC samples affected relations between turbidity and SSC, requiring the use of multiple surrogate regression models over short time frames. Dissolved oxygen minimum values were recorded in two of the monitoring years, with a minimum value of 0.71 and 3.4 mg/L recorded at the onset of the drawdowns in water years 2016 and 2018, respectively. Dissolved oxygen values below 4 mg/L lasted for 1 h, suggesting a rapidly expressed chemical oxygen demand. The response of suspended sediment loads and SSC highlight the site-specific nature of reservoir drawdowns, and the need for evaluation of expected sediment responses for drawdowns being considered at other locations.

Gaoping Submarine Canyon (GPSC) off southwestern Taiwan is a high energy canyon connected to a small mountain river with extremely high sediment load (∼10 kt km–2 y–1). Due to heavy seasonal precipitation (>3,000 mm y–1) and high tectonic activity in the region, the GPSC is known for active sediment transport processes and associated submarine geohazards (e.g., submarine cable breaks). More importantly, strong internal tides have been recorded in the GPSC to drive head-ward, bottom-intensified currents, which result in sediment erosion and resuspension in response to the tidal cycles. To understand the effects of extreme physical conditions on marine nematodes, we sampled the surface sediments along the thalweg of upper GPSC and adjacent slope (200–1,100 m) using a multicorer in the summer and fall of 2015. We found that the nematode species, functional, trophic diversity and maturity dropped significantly in the GPSC as compared with slope communities, but the nematode abundances were not affected by the adverse conditions in the canyon. The non-selective deposit-feeding, fast colonizing nematodes (e.g., Sabatieria, Daptonema, Axonolaimus, and Metadesmolaimus) dominated the canyon seafloor. In contrast, other species of non-selective deposit feeders (Setosabatieria and Elzalia), epigrowth feeders (Craspodema), omnivores/predators (Paramesacanthion), and other species constituted the diverse nematode assemblages on the slope. We found that the strong bottom currents in the GPSC may depress the local nematode diversity by removing the organic-rich, fine-grained sediments; therefore, only the resilient or fast recovering nematode species could survive and prevail. The high species turnover with depth and between the canyon and slope habitats demonstrates that strong environmental filtering processes were the primary mechanism shaping the nematode community assembly off SW Taiwan. Between the canyon and slope, a considerable contribution of nestedness pattern also indicates some degree of local extinction and dispersal limitation in the dynamic GPSC.

The margins of submarine channels are characterized by deposits that fine away from the channel thalweg. This grain‐size trend is thought to reflect upward fining trends in the currents that formed the channels. This assumption enables reconstruction of turbidity currents from the geologic record, thereby providing insights into the overall sediment load of the system. It is common to assume that the density structure of a turbidity current can be modelled with simple diffusion models, such as the Rouse equation. Yet the Rouse equation was developed to describe how particles should be distributed through the water column in open‐channel flows, which fundamentally differ from turbidity currents in terms of their flow structure. Consequently, a rigorous appraisal of the Rouse model in deep‐marine settings is needed to validate the aforementioned flow reconstructions. The present study addresses this gap in the literature by providing a robust evaluation of the Rouse model's predictions of vertical particle segregation in two experimental turbidity currents that differ only in terms of their initial bed slopes (4° versus 8°). The concentration profiles of the coarsest sediment, which is suspended predominantly in the lower part of the flow, is accurately reproduced by the Rouse equation. Significant mismatches appear, however, in the concentration of finer grained sediment, especially towards the top of the flow. This problem is caused by the mixing with clear water at the top of turbidity currents. Caution is therefore advised in applying a Rouse model to levee overspill and levee‐crest deposits. Nonetheless, the Rouse model shows good agreement with laboratory measurements in the lower regions of the flow and for the coarser grains that are predominantly transported in the lower sections of submarine channels.

Submarine canyon deposits have drawn attention due to their significance on source-to-sink analysis and hydrocarbon exploration. High-resolution 2-D and 3-D seismic and exploration well data recently collected in the Ledong-Lingshui segment of the Qiongdongnan Basin are used to investigate the depositional architecture of the palaeoslope-parallel Central Canyon, which is distinct from other slope-perpendicular canyons. This study indicates that the canyon developed along the thalweg of a multiple stepped palaeotopography with a slope-parallel descending trend eastwards. The location of the thalweg is controlled by regional tectonics and progradational slope clinoforms in the western Qiongdongnan basin. Geographic changes in an extending direction and slope gradient of the palaeotopography resulted in variations in the depth and width of the canyon. Analysis of the canyon infillings indicates multiple sediment sources including an axial sediment source from the Central Vietnam and the western Hainan Island and a canyon-side source from the northern slope of the Qiongdongnan basin. Provenance study shows that the former source supplied relatively coarse-grained turbidites and the later supplied fine-grained mass transport deposits (MTD). Most of such MTDs originated from the northern slope of the basin. Evolution of the Central Canyon can be classified into three stages. Stage 1 is characterised by significant incisions that are responsible for the formation of the canyon. Subsequently or contemporaneously, the sharp bend at the beginning of the middle segment of the canyon likely resulted in lateral erosion, which triggered large-scale and small-scale canyon margin failures in the middle and lower segments of the canyon, respectively. The subsequent early filling stage (Stage 2) refers to the deposition of turbidites supplied by the axial sediment source. However, the morphology of the stepped thalweg slope resulted in sediment bypass in the upper segment of the Central Canyon. During the late filling stage (Stage 3), MTDs supplied by the canyon-side sediment source were dominated, and interbedded with turbidite deposits. The deposition of the MTDs resulted in the sharp decreases in canyon accommodation space and the abrupt southeastwards stepping of the deepest part of the canyon. Moreover, complex interactions between debris-flows and turbidity-flows occurred during this stage. Such variations in architecture of the canyon were controlled by multiple sediment supplies and topographic changes. The proposed conceptual model of canyon infilling and the resulting stratigraphic architecture could be applied in other analogous canyons for hydrocarbon exploration.

The evolution of the channel thalweg and pool-riffle sequences is a critical feature of morphological adjustments in fluvial systems and might have important effects on aquatic habitat quality, channel stability and navigation. Based on the 220 cross-sectional profiles from 2002 to 2016, residual depths and spatial statistical analyses were used to quantitatively analyse the vertical variations and longitudinal distribution of the pool-riffle sequences and channel thalweg in the Yichang-Chenglingji Reach (YCR) of the Middle Yangtze River, which consists of the Yizhi-Zhijiang, Shashi-Gongan and Shishou-Jianli sub-reaches. The results showed that the dramatic reduction in the sediment loads following the construction of the Three Gorges Dam (TGD) has caused significant adjustments in the size, morphological diversity and spatial distribution of pool-riffle structures in the YCR. The average depth of the pools in the Yizhi-Zhijiang Reach and the Shishou-Jianli Reach overall exhibited a nonlinear growth trend, with the average residual depth increasing by 1.4 m and 1.9 m, respectively. However, the average residual depth in the Shashi-Gongan Reach showed a decreasing trend, with a reduction of approximately 1.2 m. The longitudinal lengths of the riffles in all three reaches contracted to different degrees, and the specific percentage of longitudinal river length occupied by riffles (total riffle length/total sub-reach length) decreased by 8%, 9.6% and 5.9% in the three sub-reaches from upstream to the downstream. In addition, the morphological diversity of the thalweg elevation in the Shashi-Gongan Reach and Shishou-Jianli Reach weakened after the construction of the TGD, while the bed topography in the Yizhi-Zhijiang Reach became more complex and diverse. Moreover, the pool-riffle structures in the YCR were rearranged and distributed regularly following the initiation of the TGD operation, although the variation characteristics of the average pool spacing in the three sub-reaches were quite different. Quantitative relationship between the average residual depth and the previous four-year hydrological condition was proposed for the three sub-reaches based on the Delayed Response Model (DRM) and verified using the measurements from 2013 to 2016.

Instream wood is a natural phenomenon that signifi cantly influences a function of the fluvial system in forested river basins and represents a natural part of this system. However, the majority of these relatios between instream wood and fluvial ecosystems were investigated in mountain streams or gravel-bed rivers and we are lacking field data from lowland meandering rivers. This study deals with the complex assessment of instream wood on morphology and sediment parameters including the content of organic material at spatially detailed scale of the selected bend of the Odra River. Within the site of interest (66.5 river km), 12 instream wood pieces were identified (lenght ≥ 1 m, diameter ≥ 10 cm). Their presence significantly affected river morphology, when they were the main initiator of pool formation. In total, 28 samples of surface bed sediments were collected from the thalweg, bars and lateral pool. In laboratory, samples were subjected to grain size analysis (sieving method) and loss on ignition (LOI) to obtain content of organic matter. The majority of bed sediment samples were coarse grained and these samples were located in the thalweg. The main component of these samples is gravel, often supplemented with admixtures of finer fractions. The analysis of data did not show the effect of instream wood on the deposition of organic matter in bed sediments. One of the reasons may be the presence of coarse grained material which generally contains a small amount of organic matter.

This study uses continuous-recording load cell pressure sensors in four, high-elevation (1500–1800 m), Sierra Nevada headwater streams to collect high-temporal-resolution, bedload-movement data for investigating the channel bed movement patterns within these streams for water years 2012–2014. Data show an annual pattern where channel bed material in the thalweg starts to build up in early fall, peaks around peak snow melt, and scours back to baseline levels during hydrograph drawdown and base flow. This pattern is punctuated by disturbance and recovery of channel bed material associated with short-term storm events. A conceptual model, linking sediment sources at the channel margins to patterns of channel bed fill and scour in the thalweg, is proposed building on the results of Martin et al. (2014). The material in the thalweg represents a balance between sediment supply from the channel margins and sporadic, conveyor-belt-like downstream transport in the thalweg. The conceptual model highlights not only the importance of production and transport rates but also that seasonal connectedness between the margins and thalweg is a key sediment control, determining the accumulation rate of sediment stores at the margins and the redistribution of sediment from margins to thalweg that feeds the conveyor belt. Disturbance and recovery cycles are observed at multiple temporal scales; but long term, the channel beds are stable, suggesting that the beds act as short-term storage for sediment but are in equilibrium interannually. The feasibility of use for these sensors in forested mountain stream environments is tested. Despite a high failure rate (50%), load cell pressure sensors show potential for high-temporal-resolution bedload measurements, allowing for the collection of channel bed movement data to move beyond time-integrated change measurements — where many of the subtleties of bedload movement patterns may be missed — to continuous and/or real-time measurements. This type of high-temporal-resolution data provides insight into short-term cycles of bedload movement in high gradient, forested mountain streams.

Sediment waves are common on the seafloor, but large-scale ones developed in submarine canyons are rarely reported. In this study, we document for the first time the Quaternary deep-water canyon-confined large-scale sediment waves developed on the northern South China Sea margin based on the analysis of a high-quality 3-D seismic reflection volume combined with a 2-D multichannel seismic reflection profile and three wells. Results show that the onset of these canyon-confined large-scale sediment waves can be dated back to the Penultimate Glacial Maximum (PGM)(ca.140 ka). This was the last period for the Pearl River Delta to prograde over the shelf edge. And these sediment waves have continued to develop and migrate upslope until present. The large-scale sediment waves pertaining to the PGM have a dominant down-slope asymmetrical 2-D morphology with wavelengths of 1.059–6.090 km and wave heights of 3.3–32.5 m. The present large-scale sediment waves show an up-slope asymmetrical 2-D morphology with dimensions of 0.667–5.628 km wavelength and 2.7–14.0 m wave height, which are smaller relative to those at the PGM. Grain sizes of the sediment waves at the PGM and present are interpreted to be coarse, inferred from seismic reflection data by high amplitude reflections (HARs). After a comprehensive analysis of the types of sediment-laden flows, the sediment provenances, the seafloor topography and the features of the sediment waves, these large-scale sediment waves might be interpreted as cyclic steps generated by the down-slope flowing supercritical turbidity currents and associated internal hydraulic jumps along high gradient (approximately greater than 1.28°) canyon thalwegs characterized by numerous slope breaks. The evolution of the large-scale sediment waves from partially depositional at the PGM to erosion-dominated at the present is controlled by variations of the sediment supply and the submarine slope accommodation along the canyon thalweg, which is manifested in the co-evolution of the sediment waves with the canyon. The findings of this study tell us that in addition to axial channels and mass-transport complexes, large-scale sediment waves can also develop on the canyon floor, which helps to improve our understanding of the complex canyon processes. The ‘large-scale sediment waves’ described here may be a new potential deepwater-reservoir element for the deep-water hydrocarbon exploration associated with submarine canyons.

Reservoir construction may modify the downstream flow and sediment transport, and correspondingly result in adjustments in morphodynamics of a river, especially riverbed instability. Based on hydrological datasets from 2003 to 2019 during the post-dam period using the topographic data of 57 fixed cross-sections in the Wuhan reach of the Yangtze River, we calculated the indexes representing the channel stability. Moreover, considering the effects of flow, sediment concentration, grain size of sediment, and water depth, we propose a method for calculating the equilibrium values of cumulative erosion and the lateral migrate intensity of thalweg in this paper, and the method combines with the delayed response model (DRM) to comprehensively analyze the variations in the longitudinal and lateral stabilities of the riverbed. The results revealed that the channel has been obviously eroded in the downstream reach, resulting in a 76% decrease in sediment discharge after the impoundment of the Three Gorges Reservoir (TGR). Specifically, in the past 17 years, the cumulative erosion in the Wuhan reach of the Yangtze River reached 1.72 m, while the bankfull depth increased by 1.87 m. The lateral migrate intensity of thalweg increased in response to Coriolis force, with an increase of 22.3%. Taken together, the results show that the proposed formula can effectively simulate the variation process of channel stability, and it also quantifies the extent of the influence weight of interannual flow and sediment regimes. The morphodynamics adjustments in the channel stability of our studied reach were closely related to the previous five-year flow and sediment regimes, implying that channel evolution may lag behind the changes in flow and sediment discharges.

This study uses 3D reflection seismic data to investigate how sediment gravity flows contribute to the evolution of the lower continental slope of the Myanmar margin that is part of the Bengal Fan, the largest deep-water fan system in the world. Seafloor and subseafloor data show large sediment wave fields that developed on both flanks of an extensive submarine canyon. The sediment waves exhibit asymmetric stoss and lee sides, wave lengths and heights of 850–3000 m and 25–70 m, respectively, and an upslope direction of migration. Seismic data reveals the presence of multiple fields of vertically stacked sediment waves, interbedded with units characterised by a chaotic seismic facies that accumulate mainly in the troughs of the sediment waves and can be tracked laterally to the adjacent canyons. According to their seismic facies, geometry, and internal architecture these chaotic units are interpreted as debrites. Seismic attributes extracted from different horizons indicate that the sediment waves are dominated by fine-grained sediment, while the debrites are probably associated with coarser-grained deposits. The debrites fill the troughs of the sediment waves, as well as the downstream portions of canyon thalweg, thus flattening the paleo-seafloor. The sediment waves are interpreted as cyclic steps formed by low-density turbidity currents flowing across the slope down to the basin floor, where the change in gradient favours the formation of hydraulic jumps and the transition from supercritical to subcritical flow conditions. A conceptual model for the sediment wave evolution was proposed for the study area, in the transitional environment on the lower slope, with low-density gravity flow deposits and high-density debris flow deposits alternatively accumulating on the major gravity flow conduits.

Turbidity currents are fundamental marine sedimentary processes that deliver sediments from continental margins to deep ocean basins. They develop and maintain deep-sea channels that act as principal conduits for this sediment delivery. While turbidity current channels are frequently considered as analogous to river channel systems, there are distinct differences in the mechanisms that govern their formation. This is particularly true at high-latitudes, where glacial margin processes and Coriolis effects have direct consequences on the characteristics of channel-forming turbidity currents. Understanding the latitudinal controls that influence the structure of these channels is thus a key objective of this study. The geomorphology of the glacial-fed NP-28 Channel in the Amundsen Basin of the Arctic Ocean, the highest-latitude deep-sea channel on Earth, is assessed. Compilation of available bathymetry shows that the channel follows a low-gradient, low-sinuosity path roughly paralleling the base of Lomonosov Ridge. The channel has a broad cross-section comparable to other glacially-fed and tributary-fed deep-ocean channel systems. High-resolution subbottom profiles reveal intrachannel terraces and an incised channel thalweg that were likely formed through erosion by traction-dominated currents; stratified levees and sediment waves were likely deposited by overbank flows. Coriolis deflection of channel-filling currents is evident from consistent levee asymmetry along the entire channel path. Within the channel, the channel floor geometry suggests local dominance of both Coriolis and centrifugal forces at the base of the channel; the latter in particular where the channel is deflected by external bedrock. Overall, channel morphology is largely consistent with existing models of Coriolis force effects on turbidity currents at high latitudes. This influence of Coriolis force, in addition to the presumed glacial style of the sediment input and bedrock interaction with the channel path, are considered the major factors that affected channel morphology.

The lower Yellow River (YR) reach is one of the most typical and complex river channels in the world. The impact of human activities on the evolution of its channels is an important scientific issue that needs to be explored. Based on cross-sectional channel data measured at four typical hydrological stations in the lower YR during the period from 1965 to 2015 and the accompanying annual runoff and sediment data, the variation characteristics of the longitudinal slopes of the riverbeds, the width-depth ratio of the cross-sections, and the lateral migration of thalweg points were analyzed. The river phase coefficients (ξ = W1/2/H) of the YR channel across different years were further evaluated. Furthermore, the correlation relationship between the river phase coefficient, annual discharge, and sediment concentration was established. The results show that over the last five decades, the fluctuation of the longitudinal profile of the upper and lower parts of the lower YR was more obvious than that of the middle part, and the variation range of the width of the channel cross sections and the fluctuation of the mainstream line differed in 1965–1973, 1974–1980, 1981–1985, 1986–1999, and 2000–2015. The morphological parameters of the channel cross-section (ξ) were mainly affected by the annual average discharge and sediment concentration, and a good fitting relationship was found through quantitative analyses. Large reservoirs have the function of redistributing water and sediment yields and reducing peak discharges, thereby leading to changes in the erosion-deposition patterns and river morphologies of channels.

A premise of stream restoration theory and practice is that it is often futile to attempt to restore a stream at the reach scale (101–103metres) until catchment scale problems have been addressed. This study considers reach scale restoration actions undertaken in Bryan Creek, a sand bed river in south east Australia impacted by a sediment pulse, after catchment sediment sources have been addressed. Local scale interventions, which were in-stream sand extraction, fencing to exclude stock and riparian revegetation, were evaluated by quantifying cross-section and thalweg variability, mapping in-stream and riparian vegetation and by classifying the morphology that emerged following each intervention. Following intervention channel reaches moved to one of three distinct states: simple clay bed, eroding reaches dominated byJuncus acutus, and reaches with deep pools andPhragmites australis. Boundaries between the intervention reaches were sharp, suggesting local scale interventions dominate over catchment scale processes. The magnitude and spread of variability metrics were similar between all reaches and differences in variability bore no relation to intervention type, despite the stark difference in post-intervention morphology. These findings suggest that cross-section and thalweg variability metrics are an inadequate proxy for the effectiveness of local scale interventions in accelerating the recovery of sand bed reaches from a bedload pulse. The most important implications for river managers is that local scale interventions can lead to substantial and rapid improvements in condition, and the change in condition of these reaches is almost independent of other reaches. In this case, the key to the pattern of reach scale geomorphic recovery is excluding stock from waterways so that a specific macrophyte can establish, trap sediment and develop pools.

Seasonal variability in the sediment dynamics of a Bay of Fundy tidal creek and salt marsh system was analyzed to better understand the ecomorphodynamics of a high suspended sediment concentration intertidal habitat. Data were collected over 62 tides for velocity, suspended sediment concentration, deposition, and grain size at four stations from the creek thalweg to the marsh surface. Five topographic surveys were also conducted throughout the 14-month study. Deposition rates per tide varied spatially from 56.4 g m-2 at the creek thalweg to 15.3 g m-2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0 g m-2 to 97.7 g m-2 and from 12.2 g m-2 to 19.6 g m-2 respectively. Deposition and erosion were greatest in late fall and winter. This seasonal change, led by higher suspended sediment concentrations, was observed in the creek and at the marsh bank but notably absent from the marsh edge and marsh surface. Sediments were predominantly deposited in floc form (76–83%). Because of high floc content, higher suspended sediment concentrations led to more rapid loss of sediment from suspension. With increasing sediment concentration, deposition increased in the tidal creek and at the marsh bank but not at the marsh edge or marsh surface. This suggests that in highly flocculated environments the water column clears fast enough that very little sediment remains in suspension when the water reaches the marsh and that the sediment concentration during marsh inundation is independent of the initial concentration in the creek.

River confluences and their associated tributaries are key morphodynamic nodes that play important roles in controlling hydraulic geometry and hyporheic water exchange in fluvial networks. However, the existing knowledge regarding hyporheic water exchange associated with river confluence morphology is relatively scarce. On the 14th and 15th of January, 2016, the general hydraulic and morphological characteristics of the confluent meander bend (CMB) between the Juehe River and the Haohe River in the southern region of Xi’an City, Shaanxi Province, China were investigated. The patterns and magnitudes of vertical hyporheic water exchange (VHWE) were estimated based on a one-dimension heat steady-state model while the sediment vertical hydraulic conductivity (Kv) was calculated via in situ permeameter tests. The results demonstrated that six hydrodynamic zones and their extensions were observed at the CMB during the test period. These zones were likely controlled by the obtuse junction angle and low momentum flux ratio, influencing the sediment grain size distribution of the CMB. The VHWE patterns at the test site during the test period mostly showed upwelling flow dominated by regional groundwater discharging into the river. The occurrence of longitudinal downwelling and upwelling patterns along the meander bend at the CMB was likely subjected to the comprehensive influences of the local sinuosity of the meander bend and regional groundwater discharge, and finally formed regional and local flow paths. Additionally, in dominated upwelling areas, the change in VHWE magnitudes was nearly consistent with that in Kv values, and higher values of both variables generally occurred in erosional zones near the thalweg paths of the CMB, which were mostly made up of sand and gravel. This was potentially caused by the erosional and depositional processes subjected to confluence morphology. Furthermore, lower Kv values observed in downwelling areas at the CMB were attributed to sediment clogging caused by local downwelling flow. The confluence morphology and sediment Kv are thus likely the driving factors which cause local variations in the VHWE of fluvial systems.

Flows and transport of sediment and associated chemical constituents within the lower reaches of the Athabasca River between Fort McMurray and Embarrass Airport are investigated using a two-dimensional (2D) numerical model called Environmental Fluid Dynamics Code (EFDC). The river reach is characterized by complex geometry, including vegetated islands, alternating sand bars and an unpredictable thalweg. The models were setup and validated using available observed data in the region before using them to estimate the levels of cohesive sediment and a select set of chemical constituents, consisting of polycyclic aromatic hydrocarbons (PAHs) and metals, within the river system. Different flow scenarios were considered, and the results show that a large proportion of the cohesive sediment that gets deposited within the study domain originates from the main stem upstream inflow boundary, although Ells River may also contribute substantially during peak flow events. The floodplain, back channels and islands in the river system are found to be the major areas of concern for deposition of sediment and associated chemical constituents. Adsorbed chemical constituents also tend to be greater in the main channel water column, which has higher levels of total suspended sediments, compared to in the flood plain. Moreover, the levels of chemical constituents leaving the river system are found to depend very much on the corresponding river bed concentration levels, resulting in higher outflows with increases in their concentration in the bed sediment.

Riffle-pool sequences in the thalweg paths of meandering streams are of pivotal importance to the hyporheic exchange pattern in a fluvial network, but the complex hydrodynamic, morphological, and sedimentary features of riverbed sediments increase the difficulties associated with vertical hyporheic exchange (VHE) quantification. This study applied depth-dependent radon (222Rn) and diel temperature variations to quantify VHE and residence time (tr). The study was conducted in four different hyporheic areas with riffle-pool sequences in the third-order Ghezel-Ozan River, located in north-west Iran. The mean values of temperature-derived VHE (VHET) and radon-derived VHE (VHERn) were 0.67±0.32 m/day and 0.63±0.36 m/day, respectively. Due to effects of sediment bed heterogeneity on temperature variation and 222Rn activity at downwelling and upwelling points, there were discrepancies between radon-derived (trRn) and temperature-derived residence time (trT), with mean values of 2.11±1.17 days and 1.87±1.26 days, respectively. The value of trT was well within uncertainty boundaries at a 95 percent confidence interval (pRn at the downwelling points. The analysis of vertical diel temperature, radon and electrical conductivity variations revealed subsurface water exchange to be greatly affected by larger scale regional flow. The comparison between VHET and VHERn with VHE obtained from PHAST model simulation (VHEPHAST) revealed a higher correlation between VHET and VHEPHAST (R2=0.96) than with VHERn (R2=0.76). Furthermore, vertical hydraulic conductivity (Kv) of the sediment-bed materials, calculated in situ by the permeameter test, indicated not only that Kv was up to 21 % higher in areas dominated by upward movement than at downwelling points, but also principle component analysis (PCA) demonstrated the dependence of Kv on porosity, VHE, and %sand of the stream-bed materials. This study provides evidence that vertical flux in the hyporheic zone is mainly affected by stream sinuosity and regional subsurface flow, and that the temperature method is more suitable than radon activity to quantify hyporheic exchange patterns.

The lowermost Mississippi River (LMR) is one of the largest deltaic systems in North America and one of the heavily human-manipulated fluvial river systems. Historic hydrographic surveys from the mid-1900s to the early 2010s were used to document the thalweg morphology adjustments, as well as the riffle&ndash, pool sequences. Extensive aggradation was observed during 1950s to 1960s, as the Atchafalaya River was enlarging before the completion of the Old River Control Structure (ORCS). Following the completion of the ORCS, reductions in sediment input to the LMR resulted in net degradation of the thalweg profile patterns since the mid-1960s except for the 1992&ndash, 2004 period. Different flood events that supplied sediment might be the cause of upstream aggradation from 1963&ndash, 1975 and net aggradation along the entire reach from 1992&ndash, 2004. Furthermore, the change pattern of thalweg profiles appear to be controlled by backwater effects, as well as the Bonnet Carr&eacute, spillway opening. Results from riffle&ndash, pool sequences reveal that the averaging Ws ratios (length to channel width) are 6&ndash, 7, similar to numerous previous studies. Temporal variations of the same riffles and pools reveal that aggradation and degradation might be heavily controlled by similar factors to the thalweg variations (i.e., sediment supply, backwater effects). In sum, this study examines decadal-scale geomorphic responses in a low-lying large river system subject to different human interventions, as well as natural flood events. Future management strategies of this and similar river systems should consider recent riverbed changes in dredging, sediment management, and river engineering.

A number of large rivers across the world are dredged for navigation. Dredging removes sediment from the channel, creates spoil mounds in the adjoining floodplain, and disposes sediment along the channel margin on a bar, or in open water. Retrospective studies that examine the placement and fate of this disposed material can provide guidance for other localities where dredging is practiced, and better elucidate the impacts associated with disposal at different types of sites. Many studies exist for impacts and benefits of dredging in the coastal environment, particularly to ecosystems and water quality. Few studies have examined disposal site selection and the fate of dredge spoil placed along a river corridor from a geomorphic and retrospective perspective. The Apalachicola River in Florida was dredged from the 1950s for several decades, with disposal on sand bars, in open water on the channel margins, and on the floodplain. Numerous large and small floodplain dredge-spoil mounds are a legacy from this navigation project. One of the larger mounds, disposal site 40 (DS 40), which locals call “Sand Mountain,” stands ~22 m high and extends ~10 ha in area. This disposal site originated in an artificial cutoff created in 1968–69 and grew to hold >625,000 m3 of sediment. Because this section of river was a problem for navigation when dredging was being conducted, detailed historical survey sheets with topographic and bathymetric data exist. The purpose of this paper was to analyze the local floodplain and channel geospatially, coupled with dredging and disposal data, to interpret the geomorphic changes of DS 40 and the adjoining river. Poor placement has resulted in this and other dredge spoil mounds returning sediment to the river through lateral erosion and mass wasting. Intermittent sediment pulses have altered channel morphology adjacent to the spoil mound with the channel becoming wider and shallower than originally, mean bed and thalweg elevations increasing by ~1.5 to 2 m, and width-depth ratio increasing three-fold. Although dredging of this river has not been conducted for nearly two decades, study of historical dredge spoil mounds in this reach suggest poor placement of spoil can continue to stress and alter a river in the decades that follow. Based on our findings, dredging downstream of distributaries will have poor outcomes. Dredge spoil placement in cut-off channels close to the river can be problematic because the river will regain sinuosity. Further, sites on the outer bend of meanders are more subject to erosion, and sites immediately upstream of problem or shallow areas will feed sediment to the problem reach. Findings of this study have implications to other settings, such as intensively mined rivers where unused unconsolidated material is often left on floodplains.

Large-scale mass transport complex (MTC) (usually covering hundreds to thousands of km2 with a volume of tens to hundreds of km3) resulting from slope failures, can change the overall sediment dispersal pattern in deep-water canyon. However, the causal mechanisms remain enigmatic. We investigate the large-scale MTCs in the Central Canyon that are imaged in 3D by high-resolution seismic reflection data from the northwestern South China Sea. Our results indicate that a total volume of 435.3 km3 of sediments related to large-scale mass wasting processes were dumped into the Central Canyon from its side direction between 5.7 and 4.2 Ma. This led to adjustments of the geomorphology of the thalweg slope, which changed from a simply stepped slope (2 steps separated by 1 ramp) into a complexly stepped slope (3 steps separated by 2 ramps). The adjustments are argued to be caused by the thickness variations of MTCs1-4 that also ponded the axial turbidity flows at their up-dip directions. And then, the adjusted thalweg slope controlled the formation of a multi-stage submarine fan, which is proved to be an important hydrocarbon exploration target within the Central Canyon. Besides the first step in the canyon where sediments were mainly transported downslope due to the initially large velocities of turbidity flows, the following ramps were also considered as sediment bypass zones due to the further accelerated flows. Whereas, the subsequent steps became sediment deposition dominated zones as turbidity flows decelerated abruptly. Slope overstepping resulting from high sediment supply that was controlled by the East Asia summer monsoon, together with seismicity caused by the structural inversion of the Red River Fault were responsible for the occurrences of the large-scale mass wasting processes. We highlight how the large-scale mass wasting processes change the sediment dispersal pattern in the deep-water canyon for the first time. This study is of great significant to identify the locations of hydrocarbon reservoirs and evaluate the spatial variability in depositional reservoir qualities within the deep-water canyon.

This paper assesses the size, shape and spatial organisation of organic, carbon-rich debris (peat blocks) in an upland fluvial peatland ecosystem. Peat block inventories collected in 2002 and 2012 at an alluvial reach of Trout Beck (North Pennines; United Kingdom) provide independent surveys for investigating the physical characteristics and spatial organisation of the organic debris. Peat blocks deposited along the 450 m reach represent a substantial volume of fluvially derived in-channel sediment and carbon flux at the macroscale (total peat volume 11 m3 (2002) and 17 m3 (2012)). Results show that inferred peat block transport distances depend on their size and shape. Smaller and more spherical equant shaped peat blocks are transported 1.62 and 1.72 times the distance of prolate and elongate shaped peat blocks. Downstream fining relationships provide a first-order approximation of peat block degradation rates. These degradation rates are high (up to 2 mm/m for the a-axis) and indicate considerable fine sediment release during transport. Hypsometric relations show that 73% of peat blocks are distributed within 1 channel width of the thalweg, indicating lateral organisation and a pattern of preferential deposition at the active channel margin. The local effects of obstructions from topography, roughness and slope promote peat block deposition, but given the low density of the blocks and close proximity to the flow the potential for re-entrainment is high.

Since the construction of the Deepwater Navigation Channel Project in Yangtze River Estuary 1998, it is extremely imperative to clarify the sediment dynamics in Yangtze River Estuary to reduce deposition risks in the channel. However, the sediment deposition mechanism has not been well understood. Therefore, four mechanisms of sediment deposition in Yangtze River Estuary were simulated and another one was analyzed through discussion in this research. First, a two-dimensional mathematical model of Yangtze River Estuary and four conceptual estuary models were established. Through the comparison of the models in the flow field and the sediment deposition, the characteristics of each mechanism were analyzed. The results indicated that the flow expansion, residual currents, riverbed thalweg orientation and local circulation played important roles respectively in different areas of the Yangtze River Estuary. Specifically, in the river mouth, the sediment deposition is mainly considered related to the flow expansion. While, in zones where riverine discharge and tide meet, the residual current is the primary cause. In the South Branch and the North Channel with interlaced shoals, the riverbed thalweg orientation is the major factor of the channel deposition. In the Deepwater Navigation Channel of the North Passage, the local circulation is the main cause. Finally, the sediment density current was discussed and concluded to be one of the causes leading to the sediment deposition in the North Passage. It was clear that the conceptual estuary models can be applied in explaining the mechanisms of the sediment deposition qualitatively.

Recognizing the heterogeneity of hydraulic conductivity and hyporheic flow is critical for understanding contaminant transfer and biogeochemical and hydrological processes involving streams and aquifers. In this study, the heterogeneity of hydraulic conductivity and Darcian flux in a submerged streambed and its adjacent exposed stream banks were investigated in the Beiluo River, northwest China. In the submerged streambed, Darcian flux was estimated by measurement of vertical hydraulic conductivity (K v) and vertical head gradient (VHG) using in-situ permeameter tests. On exposed stream banks, both horizontal hydraulic conductivity (K h) and K v were measured by on-site permeameter tests. In the submerged streambed, K v values gradually decreased with depth and the higher values were concentrated in the center and close to the erosional bank. Compared to the exposed stream banks, the K v values were higher in the streambed. From stream stage to the topmost layer of tested sediment, through increasing elevation, the K h values increased on the erosional bank, while they decreased on the depositional bank. The values of VHG along the thalweg illustrate that downwelling flux occurred in the deepest area while upwelling flux appeared in the other areas, which might result from the change of streambed elevation. The higher value of the Darcian flux in the submerged streambed existed near the erosional bank.

Triangular vanes are hydraulic structures used for bank protection and habitat restoration for fish and other aquatic organisms. There are limitations on the design criteria for this newly-introduced structure; therefore, the main goal of this paper is to present test results carried out on different alternatives of multiple vanes with different spaces between the vanes under different hydraulic conditions in a 90-degree mild flume bend. In addition, 3D components of flow velocity were measured in tests with and without an installed single vane. The triangular vanes create a counterclockwise secondary flow cell near the outer bank, which counteracts the clockwise main secondary flow cell in the bend for a distance of about 5le (le is the vane's effective length) downstream of the vane's position. With multiple vanes in place, the thalweg was found to be shifted toward the flume midway from the outer bank. Moreover, by increasing the space between the triangular vanes, the scour around the vane's tip and its development to the outer bank increased. Therefore, it is concluded that triangular vanes perform best when they are spaced at 5le or less.

There are many different source-to-sink dispersal systems around the world, and the Gaoping River (GPR)–Gaoping Submarine Canyon (GPSC) provides an example especially as a canyon-captured system. The GPR, a small mountainous river having an average gradient of 1:150, and the GPSC, which links the river catchment to the deep-sea basin, represent two major topographic features around SW Taiwan. Together, they constitute a terrestrial-to-marine dispersal system that has an overriding impact on the source-to-sink transport of sediment in this region. The GPSC extents from the mouth of the GPR through the shelf and slope and into the northeastern Manila Trench, a distance of about 260 km. It is a major conduit for the transport of terrestrial sediment and carbon to the South China Sea and the landward transport of particles of marine and biological origin. In the GPSC the dominant mode of suspended-sediment transport is tidal oscillations and the net direction is up-canyon. In contrast, sediment transport associated with episodic gravity-driven events is down-canyon. The steady sedimentation of the tidal regime results in hemipelagic mud across the canyon floor, whereas the gravity-driven (hyperpycnal) regime causes turbidite erosion and deposition along the canyon thalweg. Typhoon-induced river floods often lead to hyperpycnal plumes at the river mouth, which directly and indirectly ignite hyperpycnal turbidity currents in the canyon forming an effective agent for transporting large amounts of terrestrial organic material (modern and fossil carbon) to the South China Sea basin. Therefore, the GPR–GPSC represents a source-to-sink system in which terrestrial sediment in a mountainous catchment is promptly removed and transported to the deep sea by episodic gravity flows. This is also a pathway by which modern terrestrial organic carbon is quickly and effectively delivered to the deep sea.

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974. This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts.

s Comparison of geo-referred bathymetric datasets from 2000 to 2017, at different timescales, allowed to analyze changes in the sedimentary balance and to determine patterns of morphological adjustment in the Magdalena River (South America). The Magdalena River has one of the highest sediment yields among the major rivers of the world. In addition, it is a laterally constrained river mouth due to a series of man-made structures inhibiting its morphological response. Thus, it provides an illustrative example to study siltation processes in a river mouth system where a low capacity of morphological response interacts with a high-magnitude sediment transport regime. The river mouth exhibited an overall slow-pace deepening trend between 2000 and 2017. However, it experienced cyclic shifts between erosive and siltation states, both at interannual and intra-annual scales. The rates of erosion were 310 mm yr−1 on average during the erosional stages. Whereas sedimentation rates were 293 mm yr−1 on average during the sedimentation stages. Maximum rates of erosion and sedimentation were 1450 mm yr−1 and 2625 mm yr−1, respectively. However, intra-annual rates of sedimentation/erosion were of the same magnitude or larger compared with the interannual rates. Such high rates and the rapid changes in the sedimentary balance is the result of the cumulative effect of its limited morphological response capacity and its large suspended sediment load. These rapid processes generate constraints and threats to navigation. Particularly, at intra-annual scales. Consequently, innovative intervention proposals, recognizing the rapid transitions and geomorphological adaptations, are required for managing this type of river mouths.

Dredging is a globally important aquatic system management activity, used for navigation improvement, contamination removal, aggregate production and/or flood risk mitigation. Despite widespread application, understanding of the environmental effects of some dredging types remains limited. Field campaigns in 2016 and 2017 in the River Parrett estuary, UK, therefore investigated the geomorphic and physicochemical effects of Water Injection Dredging (WID), a poorly studied hydrodynamic dredging technology. WID, applied to restore channel capacity for the maintenance of flood water conveyance in the tidal River Parrett, influenced surface elevations but not grain-size characteristics of dredged bed sediments. Topographic alterations due to the 2016 WID operation were short-lived, lasting less than 10 months, although benefits of the 2017 WID operation, in terms of volumetric change, outlasted the ≈12-month study period. Dredging had a significant impact on water physicochemistry (pH, dissolved oxygen, total suspended solids and turbidity) when comparing pre- and during-dredging conditions within the dredge reach, although time-series analysis found dredging effects were comparable in magnitude to tidal effects for some parameters. WID is typically targeted at the thalweg and not the banks, rendering the geomorphic signature of the method different to those of other, often more invasive dredging technologies (e.g. mechanical dredging methods). Further, thalweg not bankside dredging may have potential positive ecological implications, particularly where the majority of biomass is located within the channel margins, as in the tidal River Parrett. Collectively, data suggest WID can be an effective method for sediment dispersal within tidal systems although regular application may be required to maintain cross sectional areas, particularly where management precedes periods of low flows and/or high rates of sediment accumulation. In future, more work is required to better understand both the physical and ecological implications of WID as a flood risk management tool in estuaries and rivers.

In the last few decades, riverine sediment flux to the coastal zones has been decreasing globally. The sediment flux from rivers to seas is usually estimated based on the lowermost gauging stations free of tides in the rivers, for example the Yangtze River. Knowledge of decadal sediment budget and the morphological evolution in the tidal reach is still limited. Here, using historical bathymetry of the tidal reach of the Yangtze, the changes in the sediment budget and local morphology in this reach for three periods between 1970 and 2008 were investigated. In terms of input sediment flux, Period I 1970–1992 is a relatively steady period, Period II 1992–2003 is a decline period with a significant decrease in the sediment flux, and Period III 2003–2008 is a decline period after the closure of the Three Gorges Dam (TGD). The reach was nearly in equilibrium during Period I. Net erosion occurred in the reach during both the pre- and post-TGD decline periods 1992–2003 and 2003–2008, with annual erosion of approximately 65 and 37 Mt/yr, respectively. The contribution of the annual erosion in the reach to the annual sediment flux at the downstream end of the reach increased from around 16.9% during 1992–2003 to 19.1% during 2003–2008. These changes in the sediment budget are closely related to the decrease in the input sediment flux, coarsening of the sediment grain size after the closure of the TGD, and changes in the riverine water flux and tidal conditions. Characterized by a length scale of 4–32 km, local deposition/erosion patterns were quite complex, which relates to the complexity of the local geometry, bathymetry and flow condition. The horizontal location of the thalweg of the reach had limited changes during 1992–2008, likely owing to the fixed riverbank due to human intervention.

The adjustments of pool-riffle distribution along the deltaic channels in the Pearl River Delta (PRD) under the combined effects of human interventions and natural evolution are examined in the three periods, namely 1952–1977 period, 1977–1999 period, and 1999–2014 period, corresponding to the different dominant factors. The residual depths along the channel thalweg and their spatial autocorrelation are introduced to quantify the pool-riffle distribution. The results show that the average pool depths of the deltaic channels increased significantly and the percentages of longitudinal length occupied by riffles decreased significantly by large-scale sand excavation during the 1977–1999 period and by sediment input decrease during the 1999–2014 period. It is highlighted sand excavation has a greater impact on the pool-riffle distribution of the North River channel and sediment input decrease has a greater impact on the pool-riffle distribution of the West River channel. The changes in spatial autocorrelation of pool-riffle sequence reveal a dramatic decrease of the average pool spacing after sand excavation, reflecting a rearrangement in the pattern of pool-riffle sequence. Moreover, the rearrangement process of the North River channel is ahead of that of the West River channel. In addition, the inhomogeneity indexes of deltaic channels are decreasing during the study period, indicating a deterioration of aquatic habitats diversity. It is anticipated that these results are useful for river management and utilization in the PRD and other river deltas worldwide that are subject to large-scale human interventions.

Longitudinal patterns in channel complexity play an important role in our understanding of fluvial systems. The spatial scales over which external controls such as tributaries, landslides and debris flows, alter channel form and influence the broader basin-wide trends in channel form. Channel complexity across a basin has important implications for how we conceptualize basin-wide trends in channel form such as downstream hydraulic geometry and the sediment links concept. This study takes a hyperscale approach to mapping channel morphology across 200 km of the Rogue River in Southwest Oregon - that is high-resolution data over large spatial extents allowing analysis to be conducted simultaneously at and across multiple spatial scales. Using a sUAS and digital photosieving we measured gravels on exposed gravel bars for the study area. We also measured thalweg depth in the field using a singlebeam echo sounder. Slope and channel width were derived using existing GIS datasets. We used these data to create a hyperscale view of the Rogue River examining the impact of tributary and non-tributary sediment links. We used a statistical approach to determine if changes in channel form were truly unique in the context of larger-scale channel variability. Our findings show inconsistent statistical significance of the identified lateral sediment sources (LSS) between variables - e.g. a statistically significant change in slope would not necessarily correlate with an increase in unit stream power. There were many LSS which triggered statistically significant changes to channel form. We used hyperscale graphs to reveal the complexity and spatial trends associated with the relationships between a given sediment link and channel form. The results of this study illustrate the complexity with which sediment links impact channel form and the distances below a given sediment link that a detectable signal exists, if a detectable signal exists.

This study focuses on the contrasted architectural elements of the two late Pleistocene alluvial fill sequences (T1 and T2 attributed to the Late Saalian and Weichselian) of the upstream Seine River alluvial plain, la Bassee, and constitutes the first estimation of the sediment volumes associated to these glacial cycles along this river. Estimations of the incised valley and valley fill spatial distribution are obtained by cokriging of data from 546 wells collected from the French national repository and a digital elevation model (DEM) of the present topography. The well stratigraphic attribution is given by the homogenised geomorphological map of la Bassee. We calculated the sediment volume of eroded material for the T1 sequence to be 1.70 109 m3 and of valley fill deposits to be 1.18 109 m3. This corresponds to a mean thickness of 6.1 m. By contrast, volumes of the T2 sequence are one-third smaller, 1.23 109 m3. The T2 mean thickness is very similar with 5.8 m. The alluvial plain is divided into four reaches representing a typical assemblage of the T1 and T2 incision style and valley fill deposits. Along the downstream reaches (1 - Montereau-Bazoches, 2 - Bazoches-Grisy), the T2 proportion per kilometer are close to the alluvial plain mean value (reach 1: 76%; reach 2: 69%, la Bassee: 71%), the T2 mean incised valley width corresponds to three quarters of the T1 (4 km). In the middle upstream reach (3 - Grisy-Courceroy), the T2 proportion is the largest (92%), the T2 mean incised valley width is the largest and comparable to the T1 (5.5 km). Along the upstream reach (4 - Courceroy-Romilly), the T2 proportion is the lowest (53%) and the T2 incised valley (2 km wide) is enclosed within the T1 incised valley (4 km wide). The rate of exported material, though, is the same for each sequence in any reach (31%). The spatial pattern of the T1-T2 deposits coincides largely with the presence of a knickpoint along the T2 paleothalweg (downstream part of reach 4, slope face 1.5‰), suggesting that the knickpoint had a large influence on the fluvial processes. The toe of the knickpoint (reaches 3 to 1, slope 0.24‰) was the site of the most efficient erosion of the T1 deposits. We propose that the change of slope favored diversion of the channel path. Upstream of the knickpoint face (upstream part of reach 4, thalweg slope 0.34‰), the incision processes were dominated by incision, avulsions or multithreading with limited lateral migration as indicated by the numerous T1 relicts preserved across valley. The sediment export corresponds to a mean incision rate of 40 m/Ma during the last 300 kyr; a fairly low value for the Paris basin, suggesting a low uplift area north of the Massif Central. Such contrast between the fluvial processes during the T1 and T2 sequences is most likely to be looked for in the climatic conditions that existed at the onset of the incision as no local controls for the knickpoint location could be identified such as active faults, substrate heterogeneity, or flow modifications. The lower sediment flux and the knickpoint associated to the T2 sequence suggest a transient river profile and valley incision at the end of the climatic transition associated to the Weichselian (T2).

Submarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain-size decrease upward in the flow, and this is a fundamental factor that affects the location and grain-size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously it was difficult to link turbidity current structure to channel-levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain-size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current. This article is protected by copyright. All rights reserved.

The documented presence of the pesticide DDT (1,1,1-trichloro-2,2-bis( p -chlorophenyl)ethane) and its metabolites DDE (1,1-dichloro-2,2-bis( p -chlorophenyl)ethylene), and DDD (1,1-dichloro-2,2-bis( p -chlorophenyl)ethane), collectively referred to as ΣDDT, in sediments from the Monterey Submarine Canyon, Monterey Fan Channel and Monterey Fan indicates that during the present sea-level highstand the Monterey Fan has been an active depositional system. The presence of ΣDDT in marine sediment is an indication of the recent arrival of terrigenous fine-grained sediment because DDT was only applied on land in California between 1944 and 1972, and is being transported to the ocean in the fine-grained sediment fraction of rivers. ΣDDT was found in 81 out of 82 cores collected in water depths deeper than 3000 m in the Monterey Fan Channel and in the Monterey Fan, within a geographical extent of 250 km to the south and 210 km to the west of the Monterey Canyon head, with surface concentrations averaging 6 and 3 ng g − 1 , in the upper and lower Monterey Fan, respectively. This is the first documentation of ΣDDT presence in ocean sediments below 3000 m water depth worldwide. The estimated total amount of ΣDDT in the fan is consistent with the amount of ΣDDT discharged by the Salinas and Pajaro Rivers since DDT started being used in California, suggesting that current fine-grained sediment discharge from these rivers has been accumulating in the fan. The ratio DDE/ΣDDT, an indication of the extent of DDT degradation, provides a first-order proxy for the relative age of the sediments. DDT is found along the thalweg of the Monterey Fan channel as far as 215 km from the canyon head, indicating recent fine-grained sediment transport along the channel. In contrast, only DDE was detected in sediments on the fan, which suggests that the fan does not accumulate fine-grained sediments at a constant rate.

This paper provides novel observations linking the connections between spatially distributed bed load transport pathways, hydraulic patterns, and morphological change in a shallow, gravel bed braided river. These observations shed light on the mechanics of braiding processes and illustrate the potential to quantify coupled material fluxes using remotely sensed methods. The paper focuses upon a 300 m long segment of the Rees River, New Zealand, and utilizes spatially dense observations from a mobile acoustic Doppler current profiler (aDcp) to map depth, velocity, and channel topography through a sequence of high-flow events. Apparent bed load velocity is estimated from the bias in aDcp bottom tracking and mapped to indicate bed load transport pathways. Terrestrial laser scanning (TLS) of exposed bar surfaces is fused with the aDcp surveys to generate spatially continuous digital elevation models, which quantify morphological change through the sequence of events. Results map spatially distributed bed load pathways that were likely to link zones of erosion and deposition. The coherence between the channel thalweg, zone of maximum hydraulic forcing, and maximum apparent bed load pathways varied. This suggests that, in places, local sediment supply sources exerted a strong control on the distribution of bed load, distinct from hydraulic forcing. The principal braiding mechanisms observed were channel choking, leading to subsequent bifurcation. Results show the connection between sediment sources, pathways, and sinks and their influence on channel morphology and flow path directions. The methodology of coupling spatially dense aDcp surveys with TLS has considerable potential to understand connections between processes and morphological change in dynamic fluvial settings.

This paper reports findings from experimental studies of sediment transport and bed morphology at asymmetrical confluences with hyperconcentrated tributary flows in the upper Yellow River. The results indicate that the hyperconcentrated flow confluence can be divided into four hydraulic regions, including the backwater zone above the upstream junction corner, the maximum velocity area, the separation flow zone, and the post-confluence region downstream of the junction corner. The bed morphology also consists of four basic elements, including the sandbar in the backwater zone, the bar in the separation flow zone, the thalweg for flow conveyance and sediment transport, and bars in the reach downstream of the separation zone. The sediment load of the hyperconcentrated flow from the tributary was the most important control on fluvial processes at such confluences. The increase in deposition in the backwater zone as the sediment load increased was almost linear, and the depth of sediment deposition in the backwater zone was approximately normal in distribution. The validity of a conceptual model for discriminating the status of the backwater effect, developed earlier from field data using the relationship between the sediment load and water volume of hyperconcentrated flows, was confirmed by the experiments. Deposition in the reach downstream of the junction, sandbar height in the backwater zone, and the width and length of the separation zone bar all tended to increase as the sediment load in the tributary increased. An obvious upstream-directed density current occurred in the backwater zone when the sediment concentration of the hyperconcentrated flow exceeded a critical value. The travel distance of the density current increased as the sediment load in the tributary increased. A formula was proposed, based on sediment continuity, for estimating the deposition volume in the reach downstream of the junction. Compared with ordinary sediment-laden flow confluences, hyperconcentrated flow confluences have a sandbar in the backwater zone associated with an upstream-directed density current that may sometimes block the main channel. Hyperconcentrated flow confluences have a thalweg, and so are different from debris flow confluences, which have a fan-shaped deposit.

The contemporaneous activity and sedimentary processes in a series of inner-shelf submarine canyons located in the Lower St. Lawrence Estuary were examined using high-resolution multibeam bathymetry and backscatter data. The presence of crescent-shaped bedforms (CSBs) displaced upslope at the bottom of the canyons between 2007 and 2012 indicates that they are currently active through the remobilization of sediment by gravity flows. However, the shelf and shores of the region are characterized by the absence of sediment. Our results indicate that gravity flows currently eroding the canyon floors do not transport new material downslope coming from the shelf but rather remobilize in-situ deglacial sediments within the canyon thalweg. We suggest that slope failures and internal tides/waves are responsible for sediment remobilization in these canyons, although their role in the upslope migrating CSBs is unclear. This paper provides evidence that sediment supply is not a prerequisite for the modern activity of inner-shelf submarine canyons when processes such as slope failures and internal tides/waves are frequent enough to remobilize in-situ sediments.