Delta Growth Research Articles

Enhanced mud retention as an autogenic mechanism for sustained delta growth: Insight from records of the Lafourche subdelta of the Mississippi River

ABSTRACTMud deposition is acknowledged as a significant contributor to delta architecture, yet its role is often oversimplified as a constant parameter in models of delta formation. A better understanding of mud retention on deltas would resolve remaining questions regarding delta growth. This study explores how spatiotemporally varied mud retention facilitates sustained delta growth in defiance of the concept of autoretreat, that is, the idea that shoreline progradation rates decline as a delta grows due to the expansion of subaqueous and subaerial delta surfaces. This research is inspired by prior field observations of the river‐dominated Mississippi Delta, USA, where the shoreline of a ca 6000 to 8000 km2 subdelta prograded at a constant rate for roughly a millennium, despite its expanding delta surface, compaction and sea‐level rise. For this, a laterally averaged one‐dimensional numerical model is leveraged to test hypotheses that enhanced mud retention with time in: (i) the delta bottomset; and (ii) the delta plain (floodplain) supports a constant rate of shoreline progradation in a maturing delta. Results demonstrate that enhanced mud retention in both the bottomset and delta plain facilitates sustained delta growth. Neither component by itself can replicate the case study. Yet, with these two integrated components, the model reproduces the cross‐section and linearly prograding pattern observed in the Mississippi Delta. The findings provide an autogenic mechanism for sustained delta growth and support the importance of mud as a fundamental building block of deltas that should be incorporated in delta‐growth models of engineered river diversions.

Spatial Analysis of the Growth of the Bengawan Solo Delta, Gresik Regency, East Java Province, Indonesia 1995-2023

The objective of this research is to conduct a spatial analysis of the development of the Bengawan Solo River Delta, Gresik Regency, East Java Province from 1995 - 2023. This research method uses both re-mote sensing technology and Geographic Information Systems (GIS), such as supervised classification and overlay techniques, kappa accuracy, and direct field verification (ground check). The results of this research indicate that deltaic growth has occurred in the Bengawan Solo delta region. This is demonstrated by the increasing amount of land advancing towards the sea, also known as oloran land. Analysis was done from 1995 - 2000, 2000 - 2005, 2005 - 2010, 2010 - 2015, and 2015 - 2023 to find the development rate of this difference. Quite high growth occurred in the range of 1995 - 2000 which was an area of 472.33 ha. and in 2010 - 2015 which was an area of 423.48 ha. While the range of 2000 - 2005 is an area of 253.82 ha. and 2015 - 2020 which is an area of 259.94 ha experienced growth of no more than the growth in 1995 - 2000 and in 2010 - 2015. This area experienced land erosion from 2005 - 2010, with eroded soil spanning an area of -278.38 ha. Then it will grow once more from 2010 - 2023, totaling 517.63 hectares. The new land utilization of Delta Bengawan Solo is mostly grown from mangroves naturally and planting results (44%), some are used by the surrounding community for the expansion of land (33%), and some are still empty land (19%) as well as for other provision (1%).

The role of fluvial and tidal currents on coal accumulation in a mixed‐energy deltaic setting: Pinghu Formation, Xihu Depression, East China Sea Shelf Basin

ABSTRACTThe Eocene Pinghu Formation in the Xihu Depression of the East China Sea Shelf preserves mixed‐process deltaic deposits and contains a large number of thin coal seams. This study improves the prediction of coal seam occurrences based on facies distribution and stratigraphic architecture models of deltaic deposits, using core and wireline log datasets. Sedimentological analysis reveals four facies associations, which represent delta plain (including distributary channels and interdistributary bays), delta front, prodelta and tidal flat. These facies associations reflect and preserve the interaction of fluvial and marine processes. Delta‐plain and delta‐front deposits record progressively greater tidal influences when traced southward. Well‐log correlations show that coal‐forming mires on the tide‐influenced lower delta plain were relatively favourable for peat accumulation because the stability of the tidal channels led to a stable platform for peat accumulation on the lower delta plain. The temporal and spatial distribution of coal seams is a function of both autogenic and allogenic responses to forcing. Increased probability of frequent changes in subsidence rates and sea‐level in an active tectonic setting and erosion by channels resulted in thin single‐layer coal seams (mostly 0.5 to 1.0 m). Autogenic processes (for example, delta growth and delta lobe switching) played a significant role in the areal distribution, lateral variation in thickness (ranging from 3 to 71 m) and large cumulative thicknesses (up to 71 m) of coal seams. A general vertical decrease in coal seam thickness likely records a cooling palaeoclimate during deposition of the Pinghu Formation. By comparing delta plain processes to favourable environments of peat accumulation in modern systems with favourable mineralogical, chemical and physical conditions, it can be concluded that: (i) relatively few and discontinuous coal seams developed on the tide‐dominated delta plain generated; (ii) laterally discontinuous and ribbon‐shaped coal seams developed in tide‐influenced deltas; whereas (iii) coal seams formed in river‐dominated deltaic environments have better lateral continuity.

Sedimentary records reveal two stages of evolution of the Abandoned Yellow River Delta from AD1128 to AD1855: vertical accretion and land-forming

In AD1128, the Yellow River shifted its course from the Bohai Sea to the South Yellow Sea (SYS) due to anthropogenic dike excavation, starting the development of the Abandoned Yellow River Delta (AYRD) that lasted for more than 700 years (AD1128-1855). However, the sediment flux of the abandoned Yellow River into the sea is in a state of change due to human activities, and the growth process of the AYRD is not well understood. Here, we investigate the growth process of the AYRD and its sedimentary record characteristics over the last millennium based on three cores collected from the AYRD.The results show that the main sediment types in the AYRD are silt, mud and sandy silt. After AD1128, the grain size components in the sediments of the AYRD showed significant stage changes with the sand content first starting to decrease. The clay content increased and remained at a high percentage in the middle to late 14th century, followed by a sharp increase from the mid-16th to the mid-17th century, due to a further increase in sediment flux from the abandoned Yellow River into the sea. A slight increase in the proportion of sand content during the final stage of the transition from subaqueous delta to terrestrial delta is a distinctive feature of the sedimentary record, and this change persists for 10 ~ 90 years in different core records.This study further proposes a schematic model of the development of the AYRD: (a) before the 16th century, the sediments were deposited mainly in the estuary and nearshore, with rapid vertical accretion; (b) After the 16th century, the horizontal land formation was the main focus, and the rate of seaward extension increased rapidly. This model also reflects the following pattern: when the sediment flux from the river into the sea is certain, the rate of land formation is inversely proportional to the rate of vertical accretion. The dominant factors affecting the evolution of the AYRD are the sediment flux into the sea and initial submerged topography, with less influence from sea level changes. Hydrodynamic erosion by wave and tidal forces from the outer delta began to dominate after the interruption of sediment supply due to the Yellow River mouth northward to the Bohai Sea in AD1855. This study has important implications for understanding the growth and evolution of deltas under the influence of human activities.

An Arctic delta reduced-complexity model and its reproduction of key geomorphological structures

Abstract. Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They are the site of sediment, nutrient, and soil organic carbon discharge to the Arctic Ocean. Arctic deltas are unique globally because they are underlain by permafrost and acted on by river and sea ice, and many are surrounded by a broad shallow ramp. Such ramps may buffer the delta from waves, but as the climate warms and permafrost thaws, the evolution of Arctic deltas will likely take a different course, with implications for both the local scale and the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced-complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019), which we have reconstructed in Julia language using published information. Unlike previous models, ArcDelRCM.jl is able to replicate the ramp around the delta. We have found that the delayed breakup of the so-called “bottom-fast ice” (i.e. ice that is in direct contact with the bed of the channel or the sea, also known as “bed-fast ice”) on and around the deltas is ultimately responsible for the appearance of the ramp feature in our models. However, changes made to the modelling of permafrost erosion and the protective effects of bottom-fast ice are also important contributors. Graph analyses of the delta network performed on ensemble runs show that deltas produced by ArcDelRCM.jl have more interconnected channels and contain less abandoned subnetworks. This may suggest a more even feeding of sediments to all sections of the delta shoreline, supporting ramp growth. Moreover, we showed that the morphodynamic processes during the summer months remain active enough to contribute significant sediment input to the growth and evolution of Arctic deltas and thus should not be neglected in simulations gauging the multi-year evolution of delta features. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl, with temperature, discharge, and ice conditions consistent with RCP7–8.5. We found that the ramp features degrade on the timescale of centuries and effectively disappear in under 1 millennium. Ocean processes, which are not included in these models, may further shorten the timescale. With the degradation of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients, and carbon releases.

Through history to growth dynamics: deciphering the evolution of spatial networks

Many ramified, network-like patterns in nature, such as river networks or blood vessels, form as a result of unstable growth of moving boundaries in an external diffusive field. Here, we pose the inverse problem for the network growth—can the growth dynamics be inferred from the analysis of the final pattern? We show that by evolving the network backward in time one can not only reconstruct the growth rules but also get an insight into the conditions under which branch splitting occurs. Determining the growth rules from a single snapshot in time is particularly important for growth processes so slow that they cannot be directly observed, such as growth of river networks and deltas or cave passages. We apply this approach to analyze the growth of a real river network in Vermont, USA. We determine its growth rule and argue that branch splitting events are triggered by an increase in the tip growth velocity.

Eco-environment and coupling coordination and quantification of urbanization in Yangtze River delta considering spatial non-stationarity

Since the 21st century, the rapid development of China’s mega-city clusters has posed a major threat to the healthy and coordinated development of cities. Therefore, it is necessary to be develop the comprehend the state of coupling coordination among mega-city cluster and EEQ under mesoscale. In this study, the largest Yangtze River Delta urban agglomeration is taken as the research object, NS-RSEI is constructed to evaluate the EEQ of the Yangtze River Delta, and the coupling coordination mechanism on the long-time series of the Yangtze River Delta in recent 20 years is explored by means of spatio-temporal analysis. The outcome verify that CCD of the Yangtze River Delta growth with a strong spatial dependence from 2001 to 2020, showing a spatial distribution pattern of " East West high-low". Above all, this study shows that urbanization is the main factor determining the development of CCD. In addition, compared with the traditional remote sensing eco-environment monitoring model, NS-RSEI proposed in this study shows better ability in mesoscale environmental monitoring, and provides great convenience for mesoscale EEQ evaluation. This study fills the research gap of the interactive coupling mechanism between urbanization and eco-environment quality of mesoscale mega-city group, and provides a new perspective on the sustainable development of megacity clusters.

Theoretical Model and Solution of Dynamic Evolution in Initial Stage of Lacustrine Delta

Sediment is carried by flows into lake during the formation of lacustrine deltas. The subsequent movement of sediment-laden flow is maintained by the initial momentum at the early stage of delta growth. A theoretical model of the plane jet boundary layer in the initial stage of a lacustrine delta, including the consideration of gravity action on the slope and bottom friction, was established according to characteristics of this process, and a similarity solution method was used to solve the model. A coefficient ε was introduced that describes the characteristics of sediment-laden flow spreading along a straight line and its value is given. The theoretical expression of morphological characteristics of the initial stage was derived based on the general mathematical model of river bed evolution, and the erosion and deposition patterns were quantitatively analyzed. It was verified through experimental data that the theoretical model can be solved accurately and, can satisfactorily describe the trend in erosion and deposition and morphological characteristics of the initial stage of a delta.

Multi‐Centennial Variability of Yangtze Delta Growth Over the Last 2000 Years: Interplay of Climate and People

AbstractReconstruction of sediment accumulation in river deltas over the Holocene provides a basis for understanding the relationship between climate change, human activities and delta growth. However, variations in deposition rates on a centennial‐scale over the last 2000 years remain poorly studied for mega‐deltas. Based on optically stimulated luminescence and AMS 14C ages, we calculated deposited sediment volumes in the Yangtze River Delta (YRD) for 500‐year intervals over the last two millennia for the first time. Our results reveal that the 2.0–1.5 and 0.5–0 ka periods had higher sediment deposition rates than in 1.5–1.0 and 1.0–0.5 ka, with maximum value about two times that of the minimum. A comparison with historical records of flooding and drought events indicates that a wetter climate led to a higher sediment deposition, reflecting the role of fluvial discharge in delta growth. However, the highest sediment deposition (205 ± 29 × 106 m3/a) over the last 500 years is not only related to the wetter conditions of the Little Ice Age, but also linked to dramatic regional population growth. We suggest that human activities have predominated over natural forcing in determining the deltaic growth over the last five centuries. Taking the Medieval Climate Anomaly with the lowest deposition volume as an analog of current climate warming, and considering the decline in sediment loads due to recent damming and soil conservation, the YRD is likely to face an even more severe deficit in sediment supply and higher risk of delta destruction in the coming centuries.

Delta size variability at Lake General Carrera, Patagonia

Punctuated lake level drops at Lake General Carrera, Chile, over the last ~13 ka are recorded in multiple stacked deltas that differ in scale. Fifteen delta terrace suites are studied to examine the factors that influence deposit morphology. Surprisingly, the pronounced longitudinal precipitation gradient is inversely correlated with the observed variations in delta size. We find the drainage basins with long-term water storage in glaciers are associated with larger deltas. In these cases, the added contribution of rare, high-magnitude ice meltwater release events (> 600 m3/s) is suggested as the differential driver to enhance delta growth. The overall progradation rate is modest (0.2-1 m/yr), and appears to have slowed over time. For the youngest deltas, we estimate the range in the intermittency factor, a dimensionless measure of the frequency of floods, is 1x10-2 to 4x10-5. Our results illustrate the natural variability of delta growth in a small region (

Log-convexity and the overpartition function

Let overline{p}(n) denote the overpartition function. In this paper, we obtain an inequality for the sequence Delta ^{2}log root n-1 of {overline{p}(n-1)/(n-1)^{alpha }} which states that log(1+3π4n5/2-11+5αn11/4)<Δ2logp¯(n-1)/(n-1)αn-1<log(1+3π4n5/2)forn≥N(α),\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned}&\\log \\biggl (1+\\frac{3\\pi }{4n^{5/2}}-\\frac{11+5\\alpha }{n^{11/4}}\\biggr )< \\Delta ^{2} \\log \\ \\root n-1 \\of {\\overline{p}(n-1)/(n-1)^{\\alpha }}\\\\&< \\log \\biggl (1+\\frac{3\\pi }{4n^{5/2}}\\biggr ) \\ \\ \ ext {for}\\ n \\ge N(\\alpha ), \\end{aligned}$$\\end{document}where alpha is a non-negative real number, N(alpha ) is a positive integer depending on alpha , and Delta is the difference operator with respect to n. This inequality consequently implies log -convexity of bigl {root n of {overline{p}(n)/n}bigr }_{n ge 19} and bigl {root n of {overline{p}(n)}bigr }_{n ge 4}. Moreover, it also establishes the asymptotic growth of Delta ^{2} log root n-1 of {overline{p}(n-1)/(n-1)^{alpha }} by showing underset{n rightarrow infty }{lim } Delta ^{2} log root n of {overline{p}(n)/n^{alpha }} = dfrac{3 pi }{4 n^{5/2}}.

Features of the methodology for studying the dynamics of tidal rivers deltas in the northern regions using satellite images &#x0D; (the Northern Dvina as example)

Using satellite images from the KeyHole 1977 and Sentinel-2B 2020 satellites, the dynamics of the tidal delta of the Northern Dvina River was investigated. When choosing images, it is necessary to take into account seasonal and tidal fluctuations in the water level; images for the dry season with a close tide phase at the moments of shooting in different years were selected. The first compiled map of delta dynamics for the period 1977–2020 shows that the growth of delta, noted by researchers earlier, continues – the attachment of islands along the sea edge to it, the accumulation of sediments and an increase in the area of channel islands in the main river-branches, narrowing and disappearance of small streams on the delta islands. But to the long-term tendency of delta growth, the processes of erosion of the coast on the islands of the sea edge are added from the side of the Dvina Bay.

Interdecadal variation in sediment yield from a forested mountain basin: The role of hydroclimatic variability, anthropogenic disturbances, and geomorphic connectivity

Variation in sediment yield may reflect a signal of disturbances in the upstream landscape, modified by sediment routing. This study, conducted in a forested drainage basin in the inland Pacific Northwest, USA, sought to generate a better insight into the interdecadal variability of sediment yield in mountain landscapes in response to environmental change during the last century. To this end, we examined: (1) sediment yield fluctuations; and (2) their association with streamflow and land use changes; as well as (3) streamflow links to climate variability modes; and (4) the influence of sediment delivery from hillslope sources to streams (lateral connectivity) and its downstream routing through the stream network (longitudinal connectivity) on land use signal at the basin's outlet. Sediment yield between 1910 and 2017, estimated based on reconstructed fluvial delta growth, displayed an order of magnitude variability, which indicates a substantial geomorphic sensitivity. The interpretation of temporal patterns and an exploratory statistical analysis pointed to land use-related sediment supply changes as the primary driver of these fluctuations, dominating system behavior before changes in environmental regulations and practices in the mid-1970s. Hydroclimatically controlled streamflow variability appeared to be more prominent in the subsequent period. Our connectivity analysis suggested that a considerable portion of coarse sediment mobilized by harvest and road construction may still reside within the channel network. In light of previous research in this landscape system, we speculate that, despite limited anthropogenic pressures in the recent decades, its characteristics and behavior continue to be conditioned by land use legacies. Overall, this study contributes to the growing understanding of profound anthropogenic transformation of the earth surface. Specifically, it demonstrates that historical resource extraction may have left a lasting imprint even in relatively remote mountain landscapes. Given the ongoing rapid environmental change, such understanding is crucial for watershed management, conservation, and restoration.

Effects of water discharge on river-dominated delta growth

River-dominated deltas are commonly developed at modern bays and lakes and ancient petroliferous basins. Water discharge is an important variable at pay zone scales in river-dominated delta reservoirs, which affects deltaic sand distributions and evolutions. However, it’s unclear how it influences river-dominated delta growth. This paper integrates Delft3D simulations and modern analogs to analyze the effects of water discharge, considering growth time, sediment supply, and coupled effects of sediment properties. High water discharges lead to the formation of lobate deltas, and the water discharge of 1,000 m 3 /s is a referenced threshold value. Fine-grained, highly-cohesive sediments increase the threshold values of water discharge at which the deltas become lobate from digitate, and vice versa. For the same simulation time, high water discharges favor more rugose shorelines, more distributary channels (especially secondary distributaries), and longer and wider deltas with more land areas. However, for the same sediment supply, high water discharges have few effects on shoreline roughness and the number of distributary channels.

Does Load‐Induced Shallow Subsidence Inhibit Delta Growth?

AbstractThe ability of deltas to persist by building new land is critical to maintaining these vital ecologic environments that are often home to major economic and population centers. However, the deposition of land‐building sediment triggers load‐induced shallow subsidence which may undermine the effectiveness of natural and engineered emergent landforms. Here, we present a new method to quantify shallow subsidence in a 6,000–8,000 km2 relict bayhead delta of the Mississippi Delta using the mouth bar to overbank stratigraphic boundary that formed near sea level, temporally constrained by optically stimulated luminescence dating. Vertical displacement rates at this boundary, averaged over 750–1,500 years, are on the order of a few mm/yr. Total subsidence scales to ∼50% of the thickness of overlying deposits, significantly greater than the 28%–35% loss estimated for inland localities underlain by peat, indicating that bay muds in the study area are more compaction‐prone than terrestrial organic‐rich deposits. Modeling shows a modest reduction of ∼13% in deltaic land‐area gain under a realistic compaction scenario for 1,000 years of simulated delta progradation, compared to a no‐compaction scenario. Our findings indicate that load‐driven compaction does not majorly hinder land‐area gain and may in fact promote long‐term growth at engineered sediment diversions through channel maintenance driven by compaction, thereby adding further support to this restoration strategy.

Benthic foraminifers from Holocene subaqueous deltas of the Western Mediterranean: Stratigraphic implications and palaeoenvironmental significance of the biofacies

To better explore the potential of Holocene subaqueous deltas as sedimentary archives, we quantitatively examined the benthic foraminiferal content of the muddy wedges belonging to three major Western Mediterranean deltas: Po Delta (N Adriatic Sea), Arno Delta (Ligurian Sea) and Rhône Delta (Gulf of Lions). The application of multivariate analyses (R-mode cluster, nMDS) on a regional dataset allowed defining six Mediterranean assemblages of taxa whose distribution, through time and space, is guided by the distance from the feeding mouths and by the balance of power between river and sea processes at the sea floor. At each site, the stratigraphic distribution of multivariate faunal data efficiently tracks the main changes in riverine fluxes and hydrodynamic levels, leading to a robust evaluation of (dis-)similarities in delta growth patterns. The dominant role of fluvial processes invariably emerges from the Po record, pointing to a river-dominated regime; whereas the foraminiferal fauna in the Arno and Rhône deltas indicates a wave-dominated regime with an intense redistribution of the river load by wind-driven currents throughout the Holocene. The distribution patterns of benthic foraminiferal assemblages in the multivariate space allow the identification of six biofacies, reflecting specific subaqueous environments that typify the Mediterranean shelves under different delta regimes: i) inner shelf; ii) deep-water mud belt; iii) shallow-water mud belt; iv) distal, river-dominated prodelta; v) distal, wave-dominated prodelta and vi) proximal prodelta. These biofacies can be a useful palaeoenvironmental tool for the interpretation of ancient, shallow-marine muddy successions belonging to the western Mediterranean stratigraphic record. The stratigraphic-based multivariate approach applied in this work can strongly support the analysis of subaqueous delta successions, developed even under different delta regimes and difficult to unravel from sedimentological features alone.

Sinuous bar fingers of digitate shallow‐water deltas: Insights into their formative processes and deposits from integrating morphological and sedimentological studies with mathematical modelling

AbstractThis paper helps to address the growing need to resolve the severe loss of deltaic lands by providing a new understanding for shallow‐water digitate delta growth. The integration of satellite image analysis of modern deltas, field studies of the Ganjiang Delta in the Poyang Lake and ‘Delft3D’ simulations further results in improved facies models for shallow‐water digitate deltas. These analyses show that shallow‐water digitate delta bar fingers are sinuous in contrast to the straight deep‐water digitate delta bar fingers. These differences are assigned to the effect of water depth on outflow hydraulics, where friction‐dominated shallow‐water delta effluents promote mouth bar deposition that then divert flow around the mouth bar, resulting in the formation of sinuous bar fingers. These effects are further strengthened by the meandering of the shallow‐water jet that increases lateral sediment transport, and by the higher flow resistance and lower gradient of the shallow‐water outflows. Our data and analyses further show differences in the morphology and deposits of the shallow‐water sinuous bar fingers, where some bar fingers develop sinuous and others meandering (with point bars) distributary channels. Lateral channel migration and point bar formation (meandering) occur as a secondary process that does not change the shape or width of the bar fingers themselves, and is suggested to be a function of slight initial differences in channel sinuosity. These differences in distributary channel morphology have a strong effect on bar finger facies distribution. Sediment cohesion is another important control on bar finger bending processes, because high cohesion promotes formation of enclosed bays, where their bank strength exceeds the centrifugal force of water flow. Lower sediment cohesion results in sinuous bar fingers without formation of enclosed bays. This work provides insights into natural and artificial shallow‐water digitate delta growth and provides new quantitative facies models for shallow‐water digitate deltas.

Projections of Global Delta Land Loss From Sea‐Level Rise in the 21st Century

AbstractRiver deltas will likely experience significant land loss because of relative sea‐level rise (RSLR), but predictions have not been tested against observations. Here, we use global data of RSLR and river sediment supply to build a model of delta response to RSLR for 6,402 deltas, representing 86% of global delta land. We validate this model against delta land area change observations from 1985–2015, and project future land area change for IPCC RSLR scenarios. For 2100, we find widely ranging delta scenarios, from +94 ± 125 (2 s.d.) km2 yr−1 for representative concentration pathway (RCP) 2.6 to −1,026 ± 281 km2 yr−1 for RCP8.5. River dams, subsidence, and sea‐level rise have had a comparable influence on reduced delta growth over the past decades, but if we follow RCP8.5 to 2100, more than 85% of delta land loss will be caused by climate‐change driven sea‐level rise, resulting in a loss of ∼5% of global delta land.

Sediment Distribution, Retention and Morphodynamic Analysis of a River-Dominated Deltaic System

River deltas have received considerable attention due to coastal land loss issues caused by subsidence, storms, and sea level rise. Improved understanding of deltaic processes and dynamics is vital to coastal restoration efforts. This paper describes the application of process-based morphodynamic models to a prograding river delta. The analysis focuses on the flow and sediment dynamics amongst the interconnected channel network of the delta. The models were validated against observations of velocity and sediment concentrations for the Wax Lake Delta (WLD) of the Atchafalaya River system in Louisiana, USA. The WLD provides an opportunity as a natural laboratory for studying the processes associated with river dominated deltaic growth. It includes a network of bifurcated channels that self-organize and dynamically adjust, as the delta grows seaward to the Gulf of Mexico. The model results for a flood event show that 47% of the flow exits the system as channelized flow and the remaining 53% exits as overbank flow. The fine sediment (silt and clay) distribution was proportional with water fluxes throughout the channel network, whereas sand distribution was influenced by geometric attributes (size, invert elevation, and alignment) of the distributary channels. The long-term deltaic growth predicted by the model compares well with the observations for the period 1998–2012. This paper provides insights on how the distribution of flow and sediment amongst the interconnected delta channels influences the morphodynamics of the delta to reach a dynamic equilibrium within this relatively young deltaic system.

Four-decades of bed elevation changes in the heavily regulated upper Atchafalaya River, Louisiana, USA

Previous studies have established that under natural conditions, alluvial river confluence zones experience channel scour followed by mid-channel bar development. Less attention is given to bed evolution downstream of large alluvial river confluences under engineered conditions, such as discharge regulation and levee confinement. Here we present four decades of morphologic changes along the 69-km uppermost Atchafalaya River, a downstream distributary of the confluence of the Mississippi River Outflow channel and the Red River. We aim to find the answer to one critical question: how does the channel bed downstream of an engineering-controlled confluence respond to regulated flow? We utilize single-beam bathymetry data collected in 1967, 1977, 1989, 1998, and 2006 to quantify riverbed deformation of the reach after the flow regulation began in 1963. Suspended sediment load and stream power during the four decadal periods are calculated. Results show that between 1967 and 2006, extensive bed degradation occurred and the average bed elevation reduced by 3.8 m. A total volume of 105 ± 26 × 106 m3 sediment was scoured from the uppermost Atchafalaya riverbed over this 40-year period, implying that channel erosion in this river reach has contributed to downstream coarse sediment delivery and associated deltaic growth in the Atchafalaya Bay. Bed aggradation only occurred during 1989–1998 in response to excess sediment input from the Mississippi River, likely attributable to the 1993 long-lasting Mississippi River flood. But the same amount of riverbed deposit eroded in the following eight years, demonstrating how quickly a large regulated river can diminish a perturbation owing to excess sediment input. These findings not only reveal the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for future modeling studies and management plans for the Atchafalaya and other large, engineered alluvial river confluences.