Mississippi River Delta Research Articles

Exploratory surveys in Taiwan of the roseau cane scale Nipponaclerda biwakoensis Kuwana (Hemiptera: Aclerdidae) and its associated parasitoids

AbstractRoseau cane (Phragmites australis (Cav). Trin. ex Steud.) is the dominant plant species of the Mississippi River Delta in Louisiana, USA, and protects the coastline from erosion and storm‐related impacts, maintaining shipping channels and oil infrastructure. Widespread dieback and thinning of P. australis were noted in the Mississippi River Delta in the fall of 2016. Invasive populations of the roseau cane scale (Nipponaclerda biwakoensis Kuwana), which is native to Asia, were found at outbreak levels in the stands. Subsequent research implicated N. biwakoensis in the diebacks. As a safe, cost‐effective and long‐term approach for managing this invasive scale, classical (importation) biological control methods are being considered. As the first step to developing biological control, research evaluating the natural enemies of the N. biwakoensis in the native range is necessary. This study investigated the population dynamics of N. biwakoensis and its associated parasitoids on P. australis in Taiwan from July to November 2019 at five study sites. Scale densities across sites increased over the growing season, peaking in September at 85.77 ± 6.36 scales/stem, which is half as dense as found in Louisiana. Mean parasitism of adult female scales across sites and season was 14.00%, while parasitism on immature scales was lower (3.00%). Four parasitoid wasp species were reared from the scale—Asytymachus lasallei Noyes and Higashiura, Boucekiella depressa Hoffer, Neastymachus japonicus Tachikawa and Aprostocetus sp. Westwood. Neastymachus japonicus was the dominant species parasitizing adult female scales, while A. lasallei was the dominant wasp species parasitizing immature scales. Astymachus lasallei and B. depressa were frequently reared from the same individual host, suggesting multiparasitism or hyperparasitism. Overall, this study provides important information on the identity and role of parasitoids of N. biwakoensis in its native range, which will aid in developing a classical biological control programme for the invasive N. biwakoensis.

Remote sensing-based structural and functional assessments of Phragmites australis diebacks in the Mississippi River Delta

• Extensive wetland changes can be caused by abiotic and biotic stressors. • Changes in wetland structure and condition can significantly impact vital navigation, flood risk, and ecosystem health services. • Remote sensing and hot spot analyses were used to measure episodic disturbance impacts on wetland condition. • The results show remotely collected data can provide enhanced spatial and temporal measures of wetland plant extent and condition as a function of landscape level dynamics. The Lower Mississippi River Delta (MRD) is dominated by Phragmites australis which provides a stabilizing force, protecting marsh communities from erosion and storm-related impacts. The MRD has experienced recent die-offs of Phragmites stands, which have coincided with a number of abiotic (hurricane and water-level) and biotic ( Phragmites scale) stressors. During this event, previously healthy stands have died or experienced stunted growth, resulting in conversion to replacement species or to open water. This study utilized remote sensing methods to (1) evaluate changes in Phragmites health and distribution in the MRD through time; (2) assess changes in plant cover and floristic quality before, during, and after the dieback event; (3) evaluate changes in landscape patterns (i.e., percentage of landscape, patch density, total edge, and aggregation index); and (4) evaluate changes in channel length and width as a function of the dieback event. Normalized Difference Vegetation Index data showed an active delta landscape with below typical plant biomass/health in periods of hurricane and dieback impacts, with recovery to health values more typical for the MRD after the dieback event. Vegetation species level assessments showed the MRD landscape experienced decreasing coverage and floristic quality during the disturbance periods, and low and nominally increasing vegetation cover and quality through the dieback and recovery periods. Landscape metrics showed similar trends, where wetland areas that experienced event-related stress during the hurricane and dieback periods, showed some level of rebound during the recovery period. And finally, bank-line change analyses showed a significant increase in the rate of shortening and widening of the Mississippi River Passes since the die-back event began. This study serves as a methodological basis for deriving vegetation trend assessments and integrating those results with landscape metrics to prioritize areas of interest; all of which are essential for effective management and mitigation of aquatic nuisance vegetation.

A modeling study of water and sediment flux partitioning through the major passes of Mississippi Birdfoot Delta and their plume structures

The Mississippi River Delta consists of six major passes and numerous small crevasses through which water and sediments are transported to the continental shelf of northern Gulf of Mexico. The water and sediment fluxes through individual passes are not routinely monitored due to the complex morphology and challenging environment. However, they are critical factors in determining the highly nonlinear near-field characteristics of freshwater plumes, fronts, possible internal hydraulic jumps, nutrient dispersal and rapid initial deposition of fine sand and mud. The flux partitioning and flow/sediment dispersal characteristics near the modern Mississippi River Delta were investigated using a high-resolution (~ 100 m horizontally) unstructured-grid, three-dimensional coupled hydrodynamic-wave-sediment numerical model. Model result analysis for the period of April–June 2010 revealed that Southwest Pass is the main conduit among the six major passes, through which 64% and 32% of the Mississippi River water and sediment, respectively, are transported to the coastal ocean. In contrast, South Pass has the lowest water and sediment fluxes among the tri-furcation channels downstream of the Head of Passes. Due to the more energetic flow at Southwest Pass compared to other passes, an elongated freshwater/sediment jet is typically present in the near-field. The average width of the plume originating from Southwest Pass, 5 km away from the mouth, is approximately 8 km, while the high sediment concentration within the plume (SSC > 10−2 kg/m3) is maintained in the upper 4 m of the water column for more than 8 km. The pattern of the plume in the far-field varies with wind direction, and the buoyant plume bends towards the coast/mid-shelf in response to downwelling/upwelling favorable winds.

Dissimilatory nitrate reduction to ammonium (DNRA) is marginal relative to denitrification in emerging-eroding wetlands in a subtropical oligohaline and eutrophic coastal delta

Nitrate (NO3−) and ammonium (NH4+) are reactive nitrogen (Nr) forms that can exacerbate eutrophication in coastal regions. NO3− can be lost to the atmosphere as N2 gas driven by direct denitrification, coupled nitrification-denitrification and annamox or retained within the ecosystems through conversion of NO3− to NH4+ via dissimilatory nitrate reduction to ammonium (DNRA). Denitrification and DNRA are competitive pathways and hence it is critical to evaluate their functional biogeochemical role. However, there is limited information about the environmental factors driving DNRA in oligohaline habitats, especially within deltaic regions where steep salinity gradients define wetland spatiotemporal distribution. Here we use the Isotope Pairing Technique to evaluate the effect of temperature (10, 20, 30 °C) and in situ soil/sediment organic matter (OM%) on total denitrification (Dtotal = direct + coupled nitrification) and DNRA rates in oligohaline forested/marsh wetlands soils and benthic sediment habitats at two sites representing prograding (Wax Lake Delta, WLD) and eroding (Barataria- Lake Cataouatche, BLC) deltaic stages in the Mississippi River Delta Plain (MRDP). Both sites receive MR water with high NO3− (>40 μM) concentrations during the year via river diversions. Denitrification rates were significantly higher (range: 18.0 ± 0.4–113.0 ± 10.6 μmol m−2 h−1) than DNRA rates (range: 0.7 ± 0.2–9.2 ± 0.3 μmol m−2 h−1). Therefore, DNRA represented on average < 10% of the total NO3− reduction (DNRA + Dtotal). Unlike denitrification, DNRA showed no consistent response to temperature. These results indicate that DNRA in wetland soils and benthic sediment is not a major nitrogen transformation in oligohaline regions across the MRDP regardless of wide range of OM% content in these eroding and prograding delta lobes.

Insecticide Resistance Monitoring of Tarnished Plant Bug (Hemiptera: Miridae) Populations in the Mid-Southern United States

Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois) (Hemiptera: Miridae), is the target for multiple insecticide applications in cotton in the mid-southern U.S. Resistance to several insecticide classes has been documented, so monitoring of resistance levels to insecticides currently used is needed before field control failures occur. Several populations were tested to estimate resistance levels for commonly used insecticides during 2017 to 2019. On average, 25 to 40% of populations were determined to be resistant to thiamethoxam, imidacloprid, and/or sulfoxaflor. There were no differences among years in mean imidacloprid and thiamethoxam LC50 values; however, the mean LC50 increased from 2017 to 2019 for sulfoxaflor. No differences in resistance were detected between the two primary agricultural regions of the Mid-South (Hills and Mississippi River Delta) for any of the insecticides. For each insecticide, the range between the most and least susceptible populations was greatest in the Hills region during 2018. Susceptibility to thiamethoxam was the most variable followed by imidacloprid and sulfoxaflor. It is important to continue monitoring for resistance because continued selection pressure is likely to lead to widespread reduced efficacy in the future.

Comparing the Yangtze and Mississippi River Deltas in the light of coupled natural-human dynamics: Lessons learned and implications for management

The Yangtze River Delta (YRD) and Mississippi River Delta (MRD) are two of the largest deltas in the world. Despite similar forcing conditions of fluvial sediment reduction and relative sea-level rise, there are significant differences in deltaic change caused by hydrodynamics and human activities. In this review, we compare the morphological changes of the two deltas at multiple spatial and temporal scales and identify the lessons learned from past engineering and management strategies. While the YRD has seen an increase of subaerial land of ~1500 km2 since 1950, the MRD has lost approximately 5000 km2 of land since the early 1930s. Extensive land reclamation and coastal shoreline embankment in the YRD have led to land gain at the expense of tidal wetland shrinkage. In contrast, flood control, waterway transportation, and petroleum industry in the MRD have resulted in levee construction and canal building, which together with subsidence and sea-level rise, have led to significant wetland loss in the MRD. The tidal forcing in the YRD is much stronger than that in the MRD, which has played a role in redistributing subaqueous sediment back into the delta plain, offsetting the negative effect of fluvial sediment reduction. In contrast with continuous population increase on the YRD, wetland loss and exposure to flooding hazards have resulted in population loss on the MRD. These comparative analyses reveal that nature and human interventions have acted together in shaping the modern deltaic morphology. Using the Driver-Pressure-State-Impact-Response (DPSIR) analysis framework, we propose a conceptual model to provide holistic understanding of the natural-human coupled delta system and management strategies, which could yield broad implications for coping with the future challenges in global deltas.

Temporal variability in the relative strength of external drivers controlling ecosystem succession in a coastal wetland near Bayou Lafourche, southeast Louisiana, USA

The coastal wetlands of Louisiana are extremely dynamic due to their location on the edge of the continent, the heavy but spatially and temporally variable influence of the discharge of the Mississippi River, and the effects of both chronic (sea-level rise, coastal retreat) and abrupt (hurricanes, lobe switching) stressors. In these coastal wetlands the timing and mode of ecological succession can result from a variety of external factors and vary from sudden to gradual. This paper uses a palynology-based multi-proxy analysis of sediment cores retrieved from a small pond on a mangrove-dominated island in Bay Champagne near Port Fourchon, Louisiana to describe and date the successive ecosystems that have covered this site since 4000 BP and applies a PCA-based interpretation to infer the drivers responsible for each shift. During the late Holocene, the most important controlling agents have been the Mississippi River delta development, sea-level rise, climatic variability, shoreline retreat, tropical cyclones, and, more recently, anthropogenic effects. These processes have progressively transformed the site from a freshwater to a saline environment, consisting of four distinct ecosystems: a Taxodium-Nyssa freshwater forest (4.0–1.3 cal yr BP), a Typha-Bacopa freshwater deltaic-lake (1.3–0.7 cal yr BP), an intermediate and brackish marsh (0.7–0.2 cal yr BP), a Spartina-dominated saline marsh (since 0.2 cal yr BP), and a short-stature Avicennia forest since the early 1900s. The Mississippi River delta cycle (lobe switching) was the dominant driver of ecosystem changes early in the site's history, while relative sea-level rise, shoreline retreat, tropical cyclones, climate, and anthropogenic effects have been the main drivers since the 19th century.

Recovery and Restoration of Biloxi Marsh in the Mississippi River Delta

The State of Louisiana is leading an integrated wetland restoration and flood risk reduction program in the Mississippi River Delta. East of New Orleans, Biloxi Marsh, a ~1700 km2 peninsula jutting 60 km north toward the State of Mississippi is one of few Delta wetland tracts well positioned to dissipate hurricane surge and waves threatening the city’s newly rebuilt hurricane flood defenses. Both its location on the eastern margin of the Delta, and its genesis as the geologic core of the shallow water St. Bernard/Terre aux Boeuf sub-delta, which was the primary Mississippi outlet for almost 2000 years, make Biloxi Marsh attractive for restoration, now that the Mississippi River Gulf Outlet deep-draft ship channel has been dammed, and 50 years of impacts from construction and operation have abated. Now, the cascade of ecosystem damage it caused can be reversed or offset by restoration projects that leverage natural recovery and increased access to suspended sediment from the Mississippi River. Biloxi Marsh is (1) geologically stable, (2) benefiting from increased input of river sediment, and (3) could be restored to sustainability earlier and for a longer period than most of the rest of the submerging Mississippi Delta. The focus of this review is on the Biloxi Marsh, but it also provides a template for regional studies, including analysis of 2D and 3D seismic and other energy industry data to explore why existing marshes that look similar on the ground or from the air may respond to restoration measures with different levels of success. Properties of inherent durability and resilience can be exploited in restoration project selection, sequencing and expenditure. Issues encountered and investigative methods applied in the Biloxi Marsh are likely to resonate across initiatives now contemplated to sustain valuable river deltas worldwide.

Linking backbarrier lacustrine stratigraphy with spatial dynamics of shoreline retreat in a rapidly subsiding region of the Mississippi River Delta

The northern Gulf of Mexico shoreline is rapidly retreating, and coastal features of the abandoned Mississippi River delta complexes are eroding and subsiding. Bay Champagne is located in the Caminada-Moreau headland, a region located west of the currently active delta that has one of the highest rates of retreat and land loss globally. As a result, the shoreline at Bay Champagne has retreated more than two kilometers in the past 130 years, causing a gradual transition from a stable, circular freshwater lake to a frequently disturbed, semi-circular backbarrier lagoon. Analyses of clastic layers in a series of sediment cores collected at this site over the past decade indicate the lake was less perturbed in the past and has become increasingly more sensitive to marine incursion events caused by tropical cyclones as the shoreline retreated. Geochemical and pollen analyses of the cores also reveal profound changes in environmental and chemical conditions at Bay Champagne over the past century. Through relating stratigraphy to spatial changes observed from satellite imagery, this study establishes a timeline of the transition of Bay Champagne from an inland lake to a backbarrier environment, and demonstrated that these changes have not occurred at a constant rate. These results provide a case-study of changing local environmental conditions in response to rapid shoreline retreat, and therefore serve as a model for future coastal changes for Louisiana and other Gulf coastal areas.

Effects of the 2017–2018 winter freeze on the northern limit of the American mangroves, Mississippi River delta plain

Global warming has led to a tropicalization of ecosystems, especially in the northern hemisphere, where freeze-sensitive plants, such as Avicennia germinans, are expanding northward, exposing tropical species to greater annual climate extremes. The effects of this driver on the boreal limits of mangroves need to be investigated for projecting the fate of mangroves in the face of global warming. Based on QuickBird, aerial/drone images, and historic air/seawater temperatures, this work analyzed the 2017–2018 winter freeze impacts on mangroves of Bay Champagne, Louisiana, USA. This winter freeze, marked by 14 days of subfreezing daily minimum temperatures, degraded ~90% (111 ha) of the studied mangroves. This macroclimatic setting caused defoliation and dry branches on Avicennia trees. Such degradation occurred mainly in mangroves established after 2004, where trees of low heights (1–1.5 m) and density between 1000 and 10,000 trees/ha were present mainly on topographically lower tidal flats (13–26 cm above mean sea-level). By contrast, healthy Avicennia trees were typically taller (1.5–2.2 m), growing at intermediate-density (4000–8000 trees/ha) stands, and occupying higher grounds (20–46 cm) in the inner part of mangrove areas. Canopy height and tree density modulated the winter freeze damages on Avicennia trees, as these factors attenuated the wind impacts along a microclimatic gradient. The microtopography of tidal flats may have also influenced the Avicennia degradation. The recovery of mangroves was rapid (~1 year) and regulated by a microclimatic and microtopographic gradient. Winter freeze impacts on mangroves will probably be attenuated as the increase in the stature and density of mangrove trees. These processes could contribute to the continued northward expansion of mangroves. However, recurring winter freezes affect mangrove productivity that is a critical factor for mangroves to keep pace with relative sea-level rise (SLR).

More-than-human Infrastructure for Just Resilience: Learning from, Working with, and Designing for Bald Cypress Trees ( Taxodium distichum ) in the Mississippi River Delta

Abstract Humans design infrastructure for human needs, with limited regard for the needs of nonhumans such as animals and plants. Humans also often fail to recognise nonhuman lifeforms such as trees as fellow engineers designers, or architects, even though the contribution of trees to ecosystem services is well established and their right to justice ought to be recognised. Studies have shown that flood-control infrastructure near the Mississippi River inadvertently left Southern Louisiana more vulnerable to coastal threats. We examine this characteristic outcome and identify infrastructural injustices in multispecies communities. Based on theories in philosophy and design supported by historical analyses, we defend the proposals to extend 1) the understanding of resilience to include more-than-human communities; and 2) the notion of justice to include non-human stakeholders. The reframing in more- than-human terms is already under way in a variety of disciplines. However, these efforts rarely extend into considerations of practical design and have attracted criticism for insufficient engagement with historical processes and the accumulations of power and responsibility. To illustrate these injustices, we trace the history of bald cypress trees ( Taxodium distichum ) in the Mississippi River Delta and show how infrastructure impacted the trees. This analysis demonstrates that designs that do not consider the needs of vulnerable stakeholders can cause harm in multispecies communities. In response, we propose that humans can work to improve infrastructural resilience by including humans and nonhumans as collaborators.

Benthic Nutrient Fluxes across Subtidal and Intertidal Habitats in Breton Sound in Response to River-Pulses of a Diversion in Mississippi River Delta

We measured benthic fluxes of dissolved nutrients in subtidal sediments and intertidal soils associated with river-pulse events from Mississippi River via the operation of a river diversion structure at Caernarvon, LA. Experiments measuring benthic fluxes in subtidal habitats were conducted during the early spring flood pulse (February and March) each year from 2002 to 2004, compared to benthic fluxes of intertidal habitats measured in February and March 2004. Nitrate (NO3−) uptake rates for subtidal sediments and intertidal soils depended on overlying water NO3− concentrations at near-, mid-, and far-field locations during river-pulse experiments when water temperatures were &gt;13 °C (NO3− removal was limited below this temperature threshold). NO3− loading to upper Breton Sound was estimated for nine river-pulse events (January, February, and March in 2002, 2003, and 2004) and compared to NO3− removal estimated by the subtidal and intertidal habitats based on connectivity, area, and flux rates as a function of NO3− concentration and water temperature. Most NO3− removal was accomplished by intertidal habitats compared to subtidal habitats with the total NO3− reduction ranging from 8% to 31%, depending on water temperature and diversion discharge rates. River diversion operations have important ecosystem design considerations to reduce the negative effects of eutrophication in downstream coastal waters.

An Examination of Compound Flood Hazard Zones for Past, Present, and Future Low-Gradient Coastal Land-Margins

Recent events worldwide demonstrate how coastal communities of integrated natural and human systems are exposed to hydrological and coastal flooding processes. Standard flood hazard assessment practices account independently for rainfall-runoff, tides, storm surge flooding and not the non-linear combination commonly defined as compound flooding. This research evaluates compound flood hazard zones for past, present, and future (c. 1890–2090) conditions of the Mississippi River Delta Plain (MRDP). The MRDP provides a low-gradient coastal land-margin representing similar landscapes around the world that are experiencing relative sea-level rise and serves as a warning beacon for our coastal settlements. A set of plausible synthetic storms and rainfall events, which account for antecedent rainfall-runoff, tropical cyclone-driven rainfall, and tropical cyclone-driven surge, are employed in a tide and surge hydrodynamic model that integrates rain over the mesh. This study demonstrates the evolution of the compound flood hazard zones from the 1890s, before major western settlement and alterations to the Mississippi River and deltaic system, to the present day and out to 2090. Furthermore, near-future projections of the compound flood hazard zones suggest that the coastal flood zone will suffer the most significant changes in coverage area due to a combination of increasing eustatic sea-level rise and alterations to the coastal land-margin during low flood events. Our results emphasize the need to establish evolution trends of compound flood hazard zones to enable more descriptive future projections under a changing climate. Such projections will aid policy-makers, stakeholders, and authorities as they pursue enhanced coastal resilience to compound flooding.

Economic Losses of Catfish to Avian Predation: A Case Report

AbstractAvian predation in catfish aquaculture has been a persistent issue throughout the history of the industry, and as production has expanded predation from piscivorous birds has intensified. Catfish ponds in the Mississippi River delta (in the Mississippi migratory flyway) provide a constant and readily accessible supply of forage for birds. Intensive foraging by fish‐eating birds has led to a specific regulatory policy and numerous on‐farm and regional management efforts. However, in 2016, legal challenges lead to recision of some federal policies and uncertainty as to allowable management, resulting in limitations on bird depredation. Estimating the extent of fish losses to avian predators is difficult, as loss estimates from farms are often confounded with disease‐ and management‐related mortalities. This study details the reported losses to birds that were observed in commercial‐scale catfish ponds at the Thad Cochran National Warmwater Aquaculture Center, Stoneville, Mississippi, during periods of limited bird management. The observed fish losses attributed to birds ranged from 33% to 95% loss in survival and potential yield losses of 4,396 to 8,889 lb/acre, increasing production costs and decreasing net returns. Net economic losses when accounting for negative net returns and lost profits ranged from US$3,518 to $4,060/acre. Losses of this magnitude on commercial fish farms are economically detrimental, especially because catfish farms lack the compensatory economic relief programs that are available in other agriculture sectors. Roost dispersal activities that are organized by federal agencies and avoiding delays in issuing bird depredation permits are vital for mitigating this persistent and growing regulatory problem in the U.S. catfish industry.

Identifying forcing agents of environmental change and ecological response on the Mississippi River Delta, Southeastern Louisiana

Freshwater wetlands on the Mississippi River delta plain, containing extensive swamps and marshes, have experienced land loss of 5197 km2 since the 1930s as the ocean has transgressed landward, resulting in landward retreat of bottomland forest, and transition of fresh to intermediate marsh. The timing and rapidity of these ecotonal transitions, and the impacts of natural and anthropogenic forces on this deltaic environment are uncertain. This study details a 448 cm sediment core retrieved from the intermediate marsh on the northern edge of Lake Salvador in southeastern Louisiana. Multi-proxy data identify six ecological shifts since 3500 BP. The ecosystem has shifted from interdistributary environment with high concentrations of such terrestrial and marine elements as Ca, Zr (3.5–3.0 cal kyr BP), to a freshwater deltaic-plain with an increase in freshwater herbs and trees (3.0–2.6 cal kyr BP), to a lacustrine environment marked by high Mn, Fe concentrations (2.6–2.2 cal kyr BP), to a swamp ecosystem with high concentrations of Zn, Br (2.2–1.4 cal kyr BP), to freshwater marsh with an increase in marsh plants (1.4–0.3 cal kyr BP), and to an intermediate marsh marked by Typha and Baccharis with elevated marine elements (since 0.3 cal kyr BP).The study identified the external forcing agents driving each deltaic environmental transition using multivariable analyses. Ecosystem dynamics are highly associated with the St. Bernard deltaic cycles, with dominant fluvial processes introducing freshwater ecosystems while forming geomorphological features such as levees, oxbow lakes, and back swamp and marsh during delta progradation. Thereafter, reduced sediment supply and decreased freshwater flow during delta transgression caused land subsidence and uneven topography. As a result, the swamp converted to marsh. Eighteenth century logging and canal development by Activities from French and Spanish settlements reduced the cypress forests and enlarged the coastal lakes.

Accounting for DEM Error in Sea Level Rise Assessment within Riverine Regions; Case Study from the Shatt Al-Arab River Region

Global elevation datasets such as the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) are the best available terrain data in many parts of the world. Consequently, SRTM is widely used for understanding the risk of coastal inundation due to climate change-induced sea level rise. However, SRTM elevations are prone to error, giving rise to uncertainty in the quality of the inundation projections. This study investigated the error propagation model for the Shatt al-Arab River region (SARR) to understand the impact of DEM error on an inundation model in this sensitive, low-lying coastal region. The analysis involved three stages. First, a multiple regression model, parameterized from the Mississippi River delta region, was used to generate an expected DEM error surface for the SARR. This surface was subtracted from the SRTM DEM for the SARR to adjust it. Second, residuals from this model were simulated for the SARR. Modelled residuals were subtracted from the adjusted SRTM to produce 50 DEM realizations capturing potential elevation variation. Third, the DEM realizations were each used in a geospatial “bathtub” inundation model to estimate flooding area in the region given 1 m of sea level rise. Across all realizations, the area predicted to flood covered about 50% of the entire region, while predicted flooding using the raw SRTM covered only about 28%, indicating substantial underprediction of the affected area when error was not accounted for. This study can be an applicable approach within such environments worldwide.

The Effect of Phragmites australis Dieback on Channel Sedimentation in the Mississippi River Delta: A Conceptual Modeling Study

Phragmites australis is a globally distributed wetland plant. At the mouth of the Mississippi River, P. australis on natural levees of the network of distributary channels appears to increase the flow in the deep draft navigation channel, which, in turn, may reduce the sedimentation and benefit the navigation dredging. For several years, P. australis has been dying in the Mississippi River’s Bird’s Foot Delta, which appears to be shortening the distributary channels and increasing the lateral flow from the remaining portions. A conceptual model based on D-FLOW FM was applied to calculate channel sedimentation in a series of idealized deltaic systems to predict the consequences of P. australis dieback and other factors that diminish the delta complexity, such as sea-level rise and subsidence, on sedimentation in the distributary channels. Channel complexity in each system, which was quantified with an index ranging from 0 to 10 that we developed. Model results indicate that sedimentation was insensitive to the channel complexity in simple deltas but was sensitive to the channel complexity in complex deltas, such as the current Mississippi River Delta with extensive P. australis. Channel sedimentation remains stable from 0 until the channel complexity index reaches 6. In more complex deltas, the sedimentation decreases rapidly as the channel complexity increases. The sedimentation is also affected by waves, river discharge, sediment concentration, grain sizes, and bed level. River managers in Louisiana may benefit from new models based on bathymetric data throughout the Bird’s Foot Delta; data on the effects of the P. australis belowground biomass on bank erodibility across a range of current velocities; and data on the effects of P. australis stem density, diameter, and height on the lateral flow across a range of river stages and tidal stages to help them decide how much to respond to Phragmites dieback. Options include increased navigation dredging, increased restoration of the channel complexity via a thin layer of sediment deposition on natural levees and the planting of more salt-tolerant vegetation on natural levees.

From the deep ocean to the coasts and estuaries through the shelf: linking coastal response to a deep blow-out

ABSTRACT # 685087As a marginal sea connected to neighboring basins through straits, the Gulf of Mexico (GoM) is dynamically and topographically complex. Physical processes are strongly influenced by the interaction of circulation in the GoM deep basin interior and in the surrounding shelf areas of diverse morphologies that include deltas, estuaries, barrier islands and marshes. This was particularly evident during the 2010 Deepwater Horizon (DwH) incident, a deep blow-out close to the Northern GoM shelves, over an area strongly affected by the brackish river plume originated from the Mississippi River Delta. The specific physical conditions are revisited, to illustrate the synergy between the evolution of the Loop Current – Florida Current system and the rapidly changing shelf and coastal currents under the influence of river runoff and winds. Each of these physical factors had been studied prior to the DwH incident, but their combined effects on hydrocarbon pathways were not known.Examples are given on what has been learned through research under the Gulf of Mexico Research Initiative (GoMRI) in the last 10 years. The focus is on transport processes in the GoM along the ocean continuum from the deep basin interior to the coastal and wetland areas, and their relevance for oil transport and fate. Post-DwH studies have advanced regarding methodologies and tools. These include multi-platform observations and data analyses, in tandem with high-resolution, data assimilative models for past simulations and predictions.Important new findings include the connectivity between remote coastal regions, as deep oceanic currents can facilitate the cross-marginal transport of materials not only locally, but regionally. This creates a broader and more challenging view for the management of coastal marine resources that should be integrated for preparedness and response. Two examples are presented on connectivity processes. First, advances in the understanding of transport rates and pathways from the Mississippi Delta to the Florida Keys. Second, new findings on how coastal circulation near Cuba influences the evolution of the Loop Current system and the oil fate from a potential oil spill in Cuban waters.The synthesis of the above findings aims to demonstrate how knowledge acquired during GoMRI can advise future planning of scientific research to aid preparedness and response not only for the GoM, but for many offshore areas of oil exploration. The goal is to advance the understanding and predictability of oil slick trajectories over pathways from the deep to the coastal environment and vice versa.

Influences on Discharge Partitioning on a Large River Delta: Case Study of the Mississippi‐Atchafalaya Diversion, 1926–1950

AbstractThe modern Mississippi River (M.R.) Delta is plumbed by the Mississippi and Atchafalaya rivers, setting water and sediment dispersal pathways for Earth’s fifth‐largest river system. The Atchafalaya River’s (A.R.) partial annexation of discharge from the M.R., particularly between 1926 and 1950, prompted warnings of a rapid river avulsion and the construction of the Old River Control Structure to regulate flow. Natural and anthropogenic causes of this discharge annexation are difficult to disentangle. Here, we develop and validate a hydrodynamic model of flow partitioning through the historic channel network. We then isolate how several key changes to the system affected discharge partitioning and stage at the A.R.‐M.R. diversion. Simulations show that erosion of the upper A.R. can account for 73% of the water discharge increase. Dredging in the lower A.R. between 1932 and 1950 can account for 35% of the water discharge increase, and was also an important control on shear stress distribution. The lower M.R. was slightly erosional during this period, and therefore hindered the A.R. discharge increase. Significant lacustrine delta deposition in A.R. had little effect on partitioning. These findings highlight the importance of A.R. enlargement processes on avulsion dynamics at this site. Given the essential nature of this river junction to the society, transportation, and commerce of the United States, improved attribution of discharge increases may lead to future management strategies that are broadly impactful.

Flooded with Error: Handling Uncertainty in SRTM for the Assessment of Sea Level Rise in the Mississippi River Delta

Digital elevation data are essential to estimate coastal vulnerability to flooding due to sea-level rise. Shuttle Radar Topography Mission (SRTM) 1 arc-second global is considered the best free global digital elevation data set available. Inundation estimates from SRTM, however, are subject to uncertainty due to inaccuracies in the elevation data. Small systematic errors in low, flat areas can generate large errors in inundation models, and SRTM is subject to positive bias in the presence of vegetation canopy, such as along channels and within marshes. In this study, we conducted an error assessment and developed a statistical error model for SRTM to improve the quality of elevation data in the Mississippi River Delta (MRD) region. Vegetation cover, SRTM elevation, and slope were found to be closely associated with SRTM error for a random sample of 10,000 small sites across the MRD region, with an ordinary least squares regression model using these variables explaining over 80 percent of the variation in error. Residuals from this model were spatially autocorrelated, and a variogram model was readily fit to them. We conclude by speculating on the utility of application of this model, developed for the MRD region, to similar near-coastal riverine regions around the world.