Subaerial Delta Research Articles

Biostratigraphy of the Eodicynodon Assemblage Zone (Beaufort Group, Karoo Supergroup), South Africa

Abstract The middle Permian Eodicynodon Assemblage Zone is the lowermost biozone of the Beaufort Group (Adelaide Subgroup, Karoo Supergroup) and occurs in the southwestern part of the main Karoo Basin. It is host to a diverse assemblage of basal therapsid genera of which Eodicynodon is the most abundant. The biozone reaches a maximum thickness of 1 100 m in the Prince Albert Road area and thins to the east and west. The biozone corresponds to the Combrinkskraal and Grootfontein members of the Abrahamskraal Formation, directly overlies the Waterford Formation of the Ecca Group, and records the earliest middle Permian terrestrial environments of Gondwana. Rocks of the biozone were deposited along the southern shoreline of the Karoo Basin in a subaerial delta plain environment as part of large-scale fan systems draining to the north and northeast within a second-order highstand systems tract.

Quantitative analysis of land transformation in a Holocene delta: An example from the Tama River lowland, central Japan

Human activities, such as the expansion of agricultural and urban land use on subaerial deltas and landfilling of subaqueous deltas have large impacts on Holocene deltas. Here, we quantified the volume of the Holocene deltaic deposits in the Tama River lowland located in the Tokyo Metropolitan Area by analyzing 3872 existing borehole sections using GIS and evaluated artificial land transformation. The total volume of deltaic deposits accumulated during the past 9000 years was estimated to be 3925 × 106 m3, of which artificially deformed near-surface material (AD) (676 × 106 m3) occupies approximately 17%. This ratio is much larger than that in the Nobi Plain (4.6%) in the Nagoya metropolitan area, suggesting that the impact of anthropogenic disturbance on the delta has been greater in Tokyo than in the Nagoya metropolitan area. Approximately 71% of AD is distributed in the seaward area, where landfills were created mainly after the Taisho period (1912–1926); the source of the AD is considered to be dredged soil, construction waste soil, and mountain sand. The remaining 29% of AD occurred in the landward area (floodplain and delta plain) and likely originates from the in situ disturbed sediments of the delta plain and floodplain in addition to earth fill. The seaward AD deposits (including redeposition by dredging within the delta) accumulated at a rate of 4.79 × 106 m3/year, which is much higher than that of the accumulation of natural deltaic deposits during the Holocene (0.36 × 106 m3/year). This clearly shows that anthropogenic forcing has become a strong influence on geomorphological and sedimentological processes within the delta.

Ephemeral rollover points and clinothem evolution in the modern Po Delta based on repeated bathymetric surveys

AbstractReconstructions of ancient delta systems rely typically on a two‐dimensional (2D) view of prograding clinothems but may miss their three‐dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high‐resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high‐resolution multi‐beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth‐bar) and muddy seaward‐dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high‐resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one‐year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut‐and‐fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.

Projections of declining fluvial sediment delivery to major deltas worldwide in response to climate change and anthropogenic stress

Deltas are resource rich, low-lying areas where vulnerability to flooding is exacerbated by natural and anthropogenically induced subsidence and geocentric sea-level rise, threatening the large populations often found in these settings. Delta ‘drowning’ is potentially offset by deposition of sediment on the delta surface, making the delivery of fluvial sediment to the delta a key balancing control in offsetting relative sea-level rise, provided that sediment can be dispersed across the subaerial delta. Here we analyse projected changes in fluvial sediment flux over the 21st century to 47 of the world’s major deltas under 12 environmental change scenarios. The 12 scenarios were constructed using four climate pathways (Representative Concentration Pathways 2.6, 4.5, 6.0 and 8.5), three socioeconomic pathways (Shared Socioeconomic Pathways 1, 2 and 3), and one reservoir construction timeline. A majority (33/47) of the investigated deltas are projected to experience reductions in sediment flux by the end of the century, when considering the average of the scenarios, with mean and maximum declines of 38% and 83%, respectively, between 1990–2019 and 2070–2099. These declines are driven by the effects of anthropogenic activities (changing land management practices and dam construction) overwhelming the effects of future climate change. The results frame the extent and magnitude of future sustainability of major global deltas. They highlight the consequences of direct (e.g. damming) and indirect (e.g. climate change) alteration of fluvial sediment flux dynamics and stress the need for further in-depth analysis for individual deltas to aid in developing appropriate management measures.

The Importance of Coarse Organic Matter and Depositional Environment to Carbon Burial Behind Dams in Mountainous Environments

AbstractThe relationship between carbon burial and sedimentation in reservoirs is unknown, contributing to uncertainty in our understanding of the net impact of dams to the global carbon budget and exposing gaps in our fundamental understanding of the transport, processing, and deposition of organic matter in fluvial and lacustrine systems. Taking opportunistic advantage of the removal of two high‐head dams, we investigate this relationship by developing a stratigraphic, process‐based framework to estimate total carbon accumulation as a function of depositional environment in the sediments of two former ~1‐km2 reservoirs on the Elwha River, Washington, USA. Former Lake Mills (upstream; completed 1927) accumulated ~330 Gg, with depositional zone average accumulation rates from 229 to 9,262 gCm2/year, while Former Lake Aldwell (downstream; completed 1913) accumulated ~91 Gg (263 to 2,414 gCm2/year). Carbon storage in both reservoirs was dominated by heterogeneous, coarse organic matter and woody debris in the coarse‐grained delta slope and relatively coarse‐grained prodelta regions of the reservoirs, with little in the gravel‐dominated, subaerial delta plains. Carbon accumulation in fine‐grained lacustrine and prodelta sediments was relatively homogeneous, but turbidity flows from the Gilbert‐style delta slope in former Lake Mills delivered significantly more carbon to the prodelta than the mouth bar‐style delta of former Lake Aldwell. C:N ratios support interpretation of most organic matter in both reservoirs as allochthonous. Sampling schemes based only on lacustrine and/or prodelta would underestimate total carbon accumulation by up to 30% in former Lake Aldwell but overestimate carbon by up to 47% in former Lake Mills.

Sediment dispersal and accumulation off the Ayeyarwady delta – Tectonic and oceanographic controls

Recent sediment dispersal and accumulation on the Northern Andaman Sea continental shelf, off the Ayeyarwady (Irrawaddy) and Thanlwin (Salween) Rivers, are investigated using seabed, water column, and high-resolution seismic data collected in December 2017. 210Pb and 137Cs derived sediment accumulation rates are highest (up to 10 cm y−1) in the mid-shelf region of the Martaban Depression, a basin that has formed on the eastern side of the N-S trending Sagaing fault, where rapid progradation of a muddy subaqueous delta is occurring. Landward of the zone of highest accumulation, in the shallow Gulf of Martaban, is a highly turbid zone where the seabed is frequently mixed to depths of ~1 m below the sediment water interface. Frequent resuspension in this area may contribute to the formation of extensive fluid muds in the water column, and consequent re-oxidation of the shallow seabed likely reduces the carbon burial efficiency for an area where the rivers are supplying large amounts of terrestrial carbon to the ocean. Sediment cores from the Gulf of Martban have a distinctive reddish brown coloration, while x-radiographs show sedimentary structures of fine silt laminations in mud deposits, which indicates strong tidal influences. The seaward part of the Martaban Depression off the modern subaqueous delta is covered by relict sediment, with no apparent connection between the modern sediment deposit and the offshore Martaban canyon. On the western side of the Sagaing fault, the “Mouths of the Ayeyarwady” represent a large promontory where the subaerial delta has prograded seaward some ~200 km along the eastern flank of the Indo-Burman range since the Holocene maximum transgression. The shelf area off the “Mouths of the Ayeyarwady” presently exhibits low sediment accumulation rates (~1 cm y−1) and a relative coarse (sandy) texture. The reduced accumulation and coarse texture in the western shelf region at the present time is attributed to frequent wave resuspension, and subsequent transport of finer river-derived sediment eastward (into the Martaban Depression) during the SW Monsoon. A mud drape (accumulation rates <1 cm/yr) is present on the northwestern part of the delta, where some sediment likely escapes the shelf to the deeper Bay of Bengal. In contrast with the Gulf of Martaban, sediments in this mud drape show olive grey coloration, while sedimentary structures are dominated by mottled sandy mud with shells, and occasional sand layers. The mud drape is likely derived from a mixture of local rivers draining westward from the Indo-Burman range, and a contribution from the Ayeyarwady system delivered during the NE monsoon. Overall, the shelf offshore the Ayeyarwady and Thanlwin rivers represents a compound subaqueous delta with marked differences from east to west that are controlled by a combination of oceanographic and tectonic factors.

Estimation of sediment compaction and its relationship with river channel distributions in the Yellow River delta, China

The geomechanical properties of the stratigraphic sequences strongly influence the land surface subsidence, as the latter can be caused by the sediment compaction. Depositional units of transitional environments such as coastal plain and deltas are characterized by a high lithological variability due to the history and complexity of the sedimentary processes. Therefore, detailed modeling of the stratigraphic architecture is necessary to better understand the land surface subsidence process and accurately estimate its rates. The frequent avulsion and rearrangement of the channel network in the modern Yellow River delta since 1855 has led to the rapid accumulation and deposition of large amounts of sediment in the delta plain. The compaction of the substratum sediment due to the new sediment load and the consolidation of younger deposits caused land surface subsidence. We used 108 sediment core data covering the delta and obtained detailed stratigraphic structural information. A detailed distribution of clay and soft clayey soils, which are the primary contributors to land subsidence, was identified. Furthermore, the compaction rates and the consolidation at time of the subaerial area and neritic area for the entire modern Yellow River delta were estimated. We also analyzed the characteristics of present land subsidence by combining a ground movement dataset obtained by high-resolution topographic surveys with Synthetic Aperture Radar (SAR) Interferometry (InSAR) of satellite images. The results show that during the early stages of the development of the active deltaic lobe, the compaction of neritic sediments was the primary contributor to initial subsidence. Furthermore, combined with the relevant sedimentary age and environment determined by previous studies, we found that the spatial variation in compaction-induced subsidence in the subaerial deltaic area over time is closely related to the historical distributions of the river channels in the modern Yellow River delta. Our findings, providing a better characterization of the spatial and time heterogeneity of the land surface subsidence of both subaerial and submarine areas, significantly improved the previous knowledge on land subsidence of the Yellow River delta. Our results also demonstrate that the subaerial delta becomes increasingly stable over time with the gradual decrease in the average subsidence rate of each deltaic lobe.

Ice and Permafrost Effects on Delta Morphology and Channel Dynamics

AbstractArctic regions are changing rapidly as permafrost thaws and sea ice retreats. These changes directly affect Arctic river deltas, but how permafrost and ice alter delta hydrology and sediment transport are not well researched. This knowledge gap limits our ability to forecast how these systems will respond to continued warming. We adapt the reduced complexity model of delta morphodynamics DeltaRCM to investigate the influences of permafrost and landfast ice on delta morphology and channel dynamics. We find that ice cover and permafrost decrease channel mobility, increase shoreline roughness, and route and deposit more sediment offshore. Ice cover also enhances overbank deposition, increasing subaerial delta elevations. Our modeling suggests that permafrost and ice loss in a warming climate could lead to less overbank and offshore deposition and more dynamic and spatially distributed fluxes of water and sediment across Arctic river deltas.

Factors Controlling Storage, Sources, and Diagenetic State of Organic Carbon in a Prograding Subaerial Delta: Wax Lake Delta, Louisiana

AbstractWax Lake Delta, southern Louisiana, is a coastal delta that formed following the dredging of a river channel in 1941 and is a field model for investigating the geomorphology, ecology, carbon dynamics, and carbon storage capacity in young prograding deltas. However, it is unknown how the transition from subaqueous to subaerial sediments affects the sources and quality of the sequestered carbon. We investigated these variations within the sediments of Wax Lake Delta using amino acid, lignin, and stable carbon isotope compositions of the organic matter (OM). A principal component analysis of these proxies highlighted variability in organic carbon (OC) composition with changes in elevation. The transition from subaqueous to subaerial sediments at 0‐cm mean lower low water is an important component of the OM composition. In addition to the changes observed for OM source and quality, the OC loadings (OC/SA; mg C/m2) also increase as the delta aggrades and accumulates sediments with loadings typical of delta topsets and mobile mud banks (OC/SA &lt; 0.4) to riverine sediments (0.5 &lt; OC/SA &lt; 1) and eventually to highly productive regions (OC/SA &gt; 1). Linking this multiproxy approach with environmental variables such as elevation provides a path for incorporating OM dynamics into geomorphic models.

Autopsy of a reservoir: Facies architecture in a multidam system, Elwha River, Washington, USA

AbstractThe 2011–2014 removal of two large dams on the Elwha River, Washington State, the largest dam removal yet completed globally, created extensive cutbank exposures of reservoir sediments, allowing the first characterization of the facies architecture of sediments through direct observation in reservoirs worldwide and providing an unparalleled opportunity to assess the relationship between environmental influences, such as changes in sediment supply, and their expression in the stratigraphic record. Using a combination of facies description from observation of 49 measured sections and &gt;100 exposures and analysis of digital elevation models and historic aerial photographs, we delineated the characteristic depositional zones of each reservoir and mapped the evolution of the subaerial delta over the life span of the reservoir. Former Lake Mills, the younger, upstream reservoir, was characterized by a tripartite, subaerial Gilbert-style delta that prograded &gt;1 km into the main reservoir from 1927 to 2011. Sediments were composed of coarse-grained topset beds, steeply dipping foreset beds, and a fine-grained, gently dipping prodelta. While individual event horizons were discernible in fine-grained sediments of former Lake Mills, their number and spacing did not correspond to known drawdown or flood events. Former Lake Aldwell, impounded from 1913 to 2011, was initially defined by the rapid progradation of a Gilbert-style, subaerial delta prior to the upstream completion of Glines Canyon Dam. However, the 1927 closure of Glines Canyon Dam upstream caused the delta to evolve to a fine-grained, mouth-bar–type delta indicative of low, finer-grained sediment. This evolution, combined with a previously unrecognized landslide deposit into the upper delta plain, suggests that understanding the exogenic influences on reservoir sedimentation is critical to interpretation and prediction of the sedimentation within individual systems.

Seismic stratigraphy and morphology of the Holocene progradational system beneath Bohai Bay, Bohai Sea: Lobate evolution of a multi-sourced subaqueous fluviodeltaic complex

Since 6 kyr BP, the subaerial deltas of the Yellow, the Yanshan and the Taihang river systems evolved along the Bohai Bay coast, forming several lobate progradational systems associated with frequent avulsions of these river systems. Their subaqueous parts consist of a multi-sourced subaqueous deltaic complex in Bohai Bay, but little is known about the complex's subsurface structure, morphology and evolutionary history. Here we investigate these characteristics using a dense, high-resolution seismic survey. Using stratigraphic superposition, seismic geometry, and the results of previous studies, we identified nine seismic units (SUs) in the post-Last Glacial Maximum stratigraphy, which are interpreted, from bottom to top, as: (1) fluvial/lacustrine sediments (SU0); (2) marine sands combined with estuarine/coastal deposits (SU1); and (3) a subaqueous fluviodeltaic complex (SUs2–8); these units correspond to the lowstand system tract, the transgressive and/or highstand system tracts, and the normal regressive system tract, respectively, and evolved as a function of eustatic fluctuations. The subaqueous fluviodeltaic complex is characterized by a discrete and spatially-confined lobate architecture, which is indicative of step-wise switching of subaqueous lobes. Two sets of progradational systems (PS), a northern one (SUs2–3) and a southern one (SUs4–8), were observed within the complex, tapering in opposite directions and bounded by an erosional unconformity (R3). These characteristics indicate that the two PS were sourced from different river systems and prograded at different times. R3 forms the base of the southern PS but cross cuts the northern PS, indicating that it is a diachronous surface marks the preexisting seafloor, which was further reworked/eroded by the progradation of the southern PS. In each PS, the internal subaqueous lobes correlated well with the onshore lobes/superlobes of the subaerial deltas, suggesting the lobate evolution of the complex is driven predominantly by alternating erosion and deposition caused by the variation in fluvial supply, which appears to be linked to the frequent avulsions of the major river systems, in addition to the multi-sourced supply. Based on the dipping geometries and the locations of the depocenters, we hypothesize that (1) the northern PS is the seaward extension of the ancient Haihe River's northern branch; (2) the bulk of southern PS in the middle and southern Bohai Bay is correlated with the principal body of the Yellow River subaqueous delta; (3) the majority of the volume of the northern PS in northern Bohai Bay consists of the modern Haihe River subaqueous delta.

Impacts of natural and human drivers on the multi-decadal morphological evolution of tidally-influenced deltas.

The world's deltas are at risk of being drowned due to rising relative sea levels as a result of climate change, decreasing supplies of fluvial sediment, and human responses to these changes. This paper analyses how delta morphology evolves over multi-decadal timescales under environmental change using a process-based model. Model simulations over 102 years are used to explore the influence of three key classes of environmental change, both individually and in combination: (i) varying combinations of fluvial water and sediment discharges; (ii) varying rates of relative sea-level rise; and (iii) selected human interventions within the delta, comprising polder-dykes and cross-dams. The results indicate that tidal asymmetry and rate of sediment supply together affect residual flows and delta morphodynamics (indicated by sub-aerial delta area, rates of progradation and aggradation). When individual drivers of change act in combination, delta building processes such as the distribution of sediment flux, aggradation, and progradation are disrupted by the presence of isolated polder-dykes or cross-dams. This suggests that such interventions, unless undertaken at a very large scale, can lead to unsustainable delta building processes. Our findings can inform management choices in real-world tidally-influenced deltas, while the methodology can provide insights into other dynamic morphological systems.

Facies architecture of Miocene subaqueous clinothems of the New Jersey passive margin: Results from IODP-ICDP Expedition 313

The preserved, laterally correlative sedimentary record of continental erosion on passive margins has been used to reconstruct past sea level changes. However, the detailed nature of a basic clinothem progradational pattern observed on many of these margins is still poorly known. This paper describes the sedimentary facies and interprets the depositional environments and the architecture of the clinothems of the New Jersey shelf to depict the origin and controls of the distribution of the sediment on the margin. We analyze 612 cores totaling 1311 m in length collected at three sites 60 km offshore Atlantic City, New Jersey, USA during IODP-ICDP Expedition 313. The three sites sampled the Lower to Middle Miocene passive margin sediments of the New Jersey shelf clinothems. We also collected wireline logs at the three sites and tied the sedimentary architecture to the geometry observed on seismic profiles. The observed sediment distribution in the clinoform complex differs from current models based on seismic data, which predict a progressive increase in mud and decrease in sand contents in a seaward direction. In contrast, we observe that the clinoforms are largely composed of muds with sands and coarser material concentrated at the rollover, the bottomset and the toe of the slope. The shelf clinothem topsets are storm-influenced mud whereas the foreset slope is composed of a mud wedge largely dominated by low density turbidites and debrites. The architecture of the clinothem complex includes a composite stack of approximately 30 m thick clinothem units each composed of four systems tracts (TST, HST, FRWST, LST) building individual T-R sequences. The presence of mud-rich facies during highstands on the topset of the clinoform, 40 to 60 km offshore from the sand-prone shoreface deposit (observed in the New Jersey onshore delta plain) and the lack of subaerial erosion (and continental depositional environments) points to a depositional model involving a subaerial delta (onshore) feeding a distant subaqueous delta. During forced regressions, shelf edge deltas periodically overstep the stacks of flood-influenced, offshore-marine mud wedges of the New Jersey subaqueous delta bringing sand to the rollover building up the large-scale shelf-prism clinothems. The clinothem complex develops on a gently dipping platform with a ramp-like morphology (apparent dip of 0.75-0.5) below mean storm wave base, in 30-50 m of water depth, 40-60 km seaward of the coastal area. Its shape depends on the balance between accommodation and sedimentation rates. Subaqueous deltas show higher accumulation rates than their subaerial counterparts and prograde three times further and faster than their contemporaneous shoreline. The increase in the intensity of waves (height, recurrence intervals) favors the separation between subaqueous and subaerial deltas, and as a consequence, the formation of a flat topset geometry, the decrease in flood events, and fluvial discharge, the overall progressive decrease in sediment grain size (from sequence m5.45, approximately 17.8-17.7 Ma, onwards) as well as the increase in sedimentation rates on the foresets of the clinoforms. All of these are recognized as preliminary signals that might characterize the entry in the Neogene icehouse world.

The Catumbela delta (SW Angola). Processes determining a history of changing asymmetry

It is presented an investigation on the morpho-sedimentary evolution of one of the largest sub-aerial delta protrusions in west Africa: the Catumbela delta. The planform geometry of this delta is characteristic of an asymmetric wave-dominated accumulation, strongly influenced by a north-directed longshore drift. Its post-glacial Holocene construction passed through morphological transformations linked with the relocation of the main fluvial channels and river mouths. Avulsion processes in the delta plain are responsible for an alternation between two major channel courses: one directed E-W and broadly coincident with today’s path and the other placed to the north with a SSE-NNW direction. Delta growth started during a period of deceleration of sea-level rise and high rainfall (c.a. 7 ka BP) and was enhanced after the 4.5 ka. BP sea-level highstand. A deflected delta configuration was promoted by the c.a. 2–3 ka BP climatic crisis, when fluvial sediment supply was reduced and sediment bypassing limited to a minimum, forcing the channel to run parallel to the coastline. This configuration can be regarded as one of the finest worldwide examples of medium-size deflected deltas. The later updrift migration in the position of the river mouth was probably determined by an increase in fluvial supply driven by an upsurge in the seasonal rainfall in the drainage area and, in particular during the last 200 years of Portuguese occupation of the territory, changes in land-use. The updrift migration, however, was irregular, being punctuated by events of downdrift shifts and two channel courses with outlets several km apart may persist during wet periods.

A Spatio-Temporal Assessment of Landcover and Coastal Changes at Wandandian Delta System, Southeastern Australia

Large numbers of people live along and depend upon the world’s coastal resources. Human modifications of the coastal zone, in combination with climate induced environmental changes, have had a major effect on the natural ecological systems. GIS analysis of remote sensed data, combined with fieldwork and laboratory tests, can be used to determine the resultant eco-geomorphic changes that need to be managed sustainably on a worldwide scale. Modelling the eco-geomorphic dynamics between 1949 and 2016 on the Wandandian Creek delta (southeastern NSW, Australia) provides a case study of management options for such coastal resources. Results from the Wandandian Creek delta show that sand/silt sediment derived from the partially (22%) modified terrestrial catchment has prograded into the wave-dominated St. Georges Basin where it is impacted by nearshore processes. Clear spatio-temporal growth of the areal extent and elevation of the deltaic levees and sandspits, with their associated mangroves and saltmarshes, has occurred over the past 65 years. Although the growth rate has fluctuated during the study period, due to flood events in 1974, 1990s and 2010, the overall subaerial and subaqueous delta area has had an average growth of 4168 m2 annually with the shoreline extending 1.451 m/year on average. This geomorphic growth has stabilised the estuarine deltaic habitats with high proportions of nutrients and organic matter, particularly within saltmarsh, mangrove, Casuarina/Juncus and other mixed native plant areas. This research shows the importance of analysing morphological changes observed on the delta that can be related to both anthropogenic modifications and natural processes to the catchment and thus should be used in the development of catchment and coastal management plans.

Dynamic controls on shallow clinoform geometry: Mekong Delta, Vietnam

Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths <14m, as expected from observed cross-shelf gradients of sedimentation. Sediment accumulation rates were highest on the upper and lower foreset beds (>4cm/yr at <10m depth, and 3–8cm/yr at ~10–20m depth) and lowest on the bottomset beds. Physically laminated sediments transitioned into mottled sediments between the upper foreset and bottomset regions. Application of a simple wave-stress model to the Mekong and several other clinoforms illustrates that shallow systems are not necessarily energy-limited, and thus rollover depths cannot be predicted solely by bed-stress distributions. In systems like the subaqueous Mekong Delta, direction of transport may have a key impact on morphology.

Sediment accumulation and budget in the Bohai Sea, Yellow Sea and East China Sea

Sediment accumulation and budget in the continental margins provides abundant information of source-sink processes from the land to the sea, including weathering, human activities and sedimentary environment. Here we show the distribution of mud areas, modern sedimentation rates and sediment budget in the Bohai Sea, Yellow Sea and East China Sea. Using grain size data of >18,000 surface sediment samples and 210Pb data from 413 sites, we identify five areas with sediments mainly composed of fine-grained fractions (mean grain size >6 φ) and find a relatively high sedimentation rates of >1.5mm/yr in the mud areas. Near the Yellow and Yangtze River deltas sedimentation reaches >95.0mm/yr. Approximately 1185.4×106tons of fine-grained sediment accumulate annually in the mud areas of the east China seas. Atmospheric deposition contributes <2% (18.37×106 tons/yr), while the riverine sediment inputs account for >75% (917×106tons/yr). The remainder comes from all other sources including coastal erosion and resuspension of bottom sediments. In addition, ~45% of the fluvial sediment supply deposits on the subaerial delta, ~40–50% is trapped on the subaqueous delta and shelf, and the remainder <5% escapes the shelf edge. The results will be a strong foundation for understanding of the transport, deposit and preservation of sediment and other relevant material (e.g. carbon and nutrient etc.) of terrestrial materials in the sea.

Stepwise morphological evolution of the active Yellow River (Huanghe) delta lobe (1976–2013): Dominant roles of riverine discharge and sediment grain size

The presently active Yellow River (Huanghe) delta lobe has been formed since 1976 when the river was artificially diverted. The process and driving forces of morphological evolution of the present delta lobe still remain unclear. Here we examined the stepwise morphological evolution of the active Yellow River delta lobe including both the subaerial and the subaqueous components, and illustrated the critical roles of riverine discharge and sediment grain size in dominating the deltaic evolution. The critical sediment loads for maintaining the delta stability were also calculated from water discharge and sediment load measured at station Lijin, the last gauging station approximately 100km upstream from the river mouth. The results indicated that the development of active delta lobe including both subaerial and subaqueous components has experienced four sequential stages. During the first stage (1976–1981) after the channel migration, the unchannelized river flow enhanced deposition within the channel and floodplain between Lijin station and the river mouth. Therefore, the critical sediment supply calculated by the river inputs obtained from station Lijin was the highest. However, the actual sediment load at this stage (0.84 Gt/yr) was more than twice of the critical sediment load (~0.35 Gt/yr) for sustaining the active subaerial area, which favored a rapid seaward progradation of the Yellow River subaerial delta. During the second stage (1981–1996), the engineering-facilitated channelized river flow and the increase in median grain size of suspended sediment delivered to the sea resulted in the critical sediment load for keeping the delta stability deceasing to 0.29 Gt/yr. The active delta lobe still gradually prograded seaward at an accretion rate of 11.9km2/yr at this stage as the annual sediment load at Lijin station was 0.55 Gt/yr. From 1996 to 2002, the critical sediment load further decreased to 0.15 Gt/yr with the sediment grain size increased to 22.5μm; however, the delta suffered net erosion because of the insufficient sediment supply (0.11 Gt/yr). In the most recent stage (2002−2013), the intensive scouring of the lower river channel induced by the dam regulation provided relatively coarser sediment, which effectively reduced the critical sediment load to 0.06 Gt/yr, much lower than the corresponding sediment load at Lijin station (~0.16 Gt/yr). Consequently, the subaerial Yellow River delta transitioned to a slight accretion phase. Overall, the evolution of the active Yellow River delta is highly correlated to riverine water and sediment discharge. The sediment supply for keeping the subaerial delta stability is inconstant and varying with the river channel morphology and sediment grain size. We conclude that the human-impacted riverine sediment discharge and grain-size composition play dominant roles in the stepwise morphological evolution of the active delta lobe.

Recent morphological changes of the Yellow River (Huanghe) submerged delta: Causes and environmental implications

The Yellow River (Huanghe) submerged delta (YRSD) has been under the threat of erosion and retreat during the Anthropocene due to dramatic climatic and anthropogenic changes in the Yellow River basin. The analysis of field data shows that over the period of 1977–2005, the changes in climate (decrease in precipitation and increase in air temperature) and human interventions (increase in water diversion projects) throughout the watershed have resulted in the sharp reductions of river flow and sediment discharges into the Bohai Sea. Consequently, over the decadal timescale, morphological evolution of the YRSD has gone through three stages: i.e. rapid accumulation (5.77×108m3/year) in 1977–1985, moderate accumulation (3.80×108m3/year) in 1986–1995 and slow accumulation (0.91×108m3/year) in 1996–2005. Climatic change within the catchment characterized by the rapid increase of air temperature contributed significantly to the transitions from the rapid accumulation to the moderate accumulation, and to the subsequent slow accumulation. The decadal morphological changes of the YRSD also show peculiar deposition/erosion characteristics over the medium timescale under river input reduction. Within the three decades, the patterns of the main sedimentary body exhibit irregular ellipses with the long axis parallel to the −5 or −10m isobaths and short axis perpendicular to the isobaths. The depocentres of the YRSD are located between the −10 and −15m isobaths close to the respective river mouths, with a high vertical accretion rate of ~1.20m/year. The time series data of annual volumetric change of the YRSD and river sediment load from 1977 to 2005 further demonstrate significant linear positive relationships between deltaic geomorphic change and fluvial input over shorter timescales (annual and 3-year). The critical sediment discharges for maintaining the deposition/erosion equilibrium state of the YRSD over the annual and 3-year timescales are found to be 1.79×108t/year and 1.29×108t/year, respectively. The analysis from the latest hydrological data (2006–2015) suggests that over the decadal timescale, the evolution state of the YRSD currently has transitioned from constructive to destructive due to the weakened fluvial input. Moreover, there exists a close quantitative link between the progradation of the Yellow River subaerial delta (YRAD) and the YRSD over the five-year timescale, indicating that every 1×108m3/year increase of the YRSD's yearly volumetric change will result in a 3.28km2/year increase of the YRAD's yearly land-accretion area, and the YRAD will reach the extension/retreat balance state when yearly morphological change of the YRSD is at 0.73×108m3/year.

Recent changing patterns of the Changjiang (Yangtze River) Estuary caused by human activities

To evaluate the controlling factors for coastline change of the Changjiang (Yangtze River) Estuary since 1974, we extracted the mean high tide line from multi-temporal remote sensing images that span from 1974 to 2014 at 2-year intervals. We chose 42 scenes to constrain the changing pattern of the Changjiang Estuary coastline, and implemented GIS technology to analyze the area change of the Changjiang (Yangtze) Subaerial Delta. Runoff, sediment discharge and coastal engineering were withal considered in the analysis of the coastline changes. The coastline has transgressed seaward since 1974, and a part of it presents inter-annual variations. The area of the Changjiang Subaerial Delta increased by 871 km2, with a net accretion rate of 21.8 km2/a. Based on the change of sediment discharge due to the major projects in the Changjiang River Basin, we divided the changing pattern of the coastline into three stages: the slow accretion stage (1974–1986), the moderate accretion stage (1987–2002), and the rapid accretion stage (2003–2014). Liner regression analysis illustrated that there is a significantly positive correlation between the area changes and sediment discharge in the Chongming Eastern Shoal and Jiuduansha. This suggested that sediment load has a fundamental effect on the evolution of the Changjiang Estuary. Construction of Deep Waterway in the North Passage of the Changjiang River (1998–2010) led to a rapid accretion in the Hengsha Eastern Shoal and Jiuduansha by influencing the hydrodynamics in North Passage. Coastal engineering such as reclamation and harbor construction can also change the morphology of the Changjiang Estuary. We defined a contribution rate of area change to assess the impact of reclamation on the evolution of Changjiang Estuary. It turned out that more than 45.3% of area increment of the Changjiang Estuary was attributed to reclamation.