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Abstract There has been considerable discussion over the past several years about the potential negative effects of nutrient loading on coastal wetland stability. In particular, there have been concerns that high nitrate concentrations can fuel denitrification that can lead to soil organic matter loss and wetland deterioration. Here we review these issues for three case studies where there have been elevated levels of dissolved inorganic nitrogen, especially nitrate, coincident with wetland deterioration. These case studies include the Breton Sound estuary that receives diverted Mississippi River water at Caernarvon LA, a freshwater assimilation wetland that receives treated municipal effluent from a wastewater treatment plant at Hammond, LA, and a tidal creek in the Plum Island Sound estuary of northeastern Massachusetts where nitrate was introduced on each flood tide during the growing season for nearly ten years. We review the physical setting, ecology and biogeochemistry of these sites and use stoichiometric calculations to estimate how much soil organic matter decomposition could be accounted for by denitrification. Results of these calculations show that denitrification rates could not have caused the observed marsh deterioration at the two LA marshes. Denitrification may play a significant role in marsh loss at the MA site, however due to the unique hydrology of this site, denitrification rates induced by very high nitrogen loading rates in concert with the hydrology of the site may play a more significant role in marsh deterioration. [ABSTRACT FROM AUTHOR]

Flood control levees cut off the supply of sediment to Mississippi delta coastal wetlands, and contribute to putting much of the delta on a trajectory for continued submergence in the 21st century. River sediment diversions have been proposed as a method to provide a sustainable supply of sediment to the delta, but the frequency and magnitude of these diversions needs further assessment. Previous studies suggested operating river sediment diversions based on the size and frequency of natural crevasse events, which were large (>5000 m 3 /s) and infrequent (active < once a year) in the last naturally active delta. This study builds on these previous works by quantitatively assessing tradeoffs for a large, infrequent diversion into the forested wetlands of the Maurepas swamp. Land building was estimated for several diversion sizes and years inactive using a delta progradation model. A benefit-cost analysis (BCA) combined model land building results with an ecosystem service valuation and estimated costs. Results demonstrated that land building is proportional to diversion size and inversely proportional to years inactive. Because benefits were assumed to scale linearly with land gain, and costs increase with diversion size, there are disadvantages to operating large diversions less often, compared to smaller diversions more often for the immediate project area. Literature suggests that infrequent operation would provide additional gains (through increased benefits and reduced ecosystem service costs) to the broader Lake Maurepas-Pontchartrain-Borgne ecosystem. Future research should incorporate these additional effects into this type of BCA, to see if this changes the outcome for large, infrequent diversions. [ABSTRACT FROM AUTHOR]

Sand transport to the Mississippi River Delta (MRD) remains sufficient to build wetlands in shallow, sheltered coastal bays fed by engineered diversions on the Mississippi River (MR) and its Atchafalaya River (AR) distributary. But suspended mud (silt & clay) flux to the coast has dropped from a mean of 390 Mt y −1 in the early 1950s, to 100 Mt y −1 since 1970. This fine-grained sediment travels deeper into receiving estuarine basins and plays a critical role in sustaining existing marshes. Virtually all of the 300 Mt y −1 of missing mud once flowed from the Missouri River (MOR) Basin before nearly 100 dams were built as part of the Pick-Sloan water development project. About 100 Mt y −1 is now intercepted by main-stem Upper MOR dams closed in 1953. But the remaining 200 Mt y −1 is trapped by impoundments built on tributaries to the Lower MOR in the 1950s and 1960s. Sediment flux during the post-dam high MOR discharge years of 1973, 1993 and 2011 approached pre-dam levels when tributaries to the Lower MOR, including the Platte and Kansas Rivers, contributed to flood flows. West bank tributaries drain a vast, arid part of the Great Plains, while those entering from the east bank traverse the lowlands of the MOR floodplain. Both provinces are dominated by highly erodible loess soils. Staunching the continued decline in MR fine-grained sediment flux has assumed greater importance now that engineered diversions are being built to reconnect the Lowermost MR to the MRD. Tributary dam bypassing in the Lower MOR basin could increase mud supply to the MRD by 100–200 Mt y −1 within 1–2 decades. Such emergency measures to save the MRD are compatible with objectives of the Missouri River Restoration and Platte River Recovery Programs to restore MOR riparian habitat for endangered species. Rapid mobilization to shunt fine-grained sediments past as many as 50 Lower MOR tributary dams in several U.S. states will undoubtedly require as much regional coordination and funding in the 21st century as the monumental effort it took to build the dams in the last century. [ABSTRACT FROM AUTHOR]

Deltas are among the most productive and economically important of global ecosystems but unfortunately they are also among the most threatened by human activities. Here we discuss deltas and human impact, several approaches to defining deltaic sustainability and present a ranking of sustainability. Delta sustainability must be considered within the context of global biophysical and socioeconomic constraints that include thermodynamic limitations, scale and embeddedness, and constraints at the level of the biosphere/geosphere. The development, functioning, and sustainability of deltas are the result of external and internal inputs of energy and materials, such as sediments and nutrients, that include delta lobe development, channel switching, crevasse formation, river floods, storms and associated waves and storm surges, and tides and other ocean currents. Modern deltas developed over the past several thousand years with relatively stable global mean sea level, predictable material inputs from drainage basins and the sea, and as extremely open systems. Human activity has changed these conditions to make deltas less sustainable, in that they are unable to persist through time structurally or functionally. Deltaic sustainability can be considered from geomorphic, ecological, and economic perspectives, with functional processes at these three levels being highly interactive. Changes in this functioning can lead to either enhanced or diminished sustainability, but most changes have been detrimental. There is a growing understanding that the trajectories of global environmental change and cost of energy will make achieving delta sustainability more challenging and limit options for management. Several delta types are identified in terms of sustainability including those in arid regions, those with high and low energy-intensive management systems, deltas below sea level, tropical deltas, and Arctic deltas. Representative deltas are ranked on a sustainability range. Success in sustainable delta management will depend on utilizing natural delta functioning and an ecological engineering approach. [ABSTRACT FROM AUTHOR]

Currently the Mississippi delta stands as a highly degraded and threatened coastal ecosystem having lost about 25% of coastal wetlands during the 20th century. To address this problem, a $50 billion, 50-year restoration program is underway. A central component of this program is reintroduction of river water back into the deltaic plain to mimic natural functioning of the delta. However, opposition to diversions has developed based on a number of perceived threats. These include over-freshening of coastal estuaries, displacement of fisheries, perceived water quality problems, and assertions that nutrients in river water leads to wetland deterioration. In addition, growing climate impacts and increasing scarcity and cost of energy will make coastal restoration more challenging and limit restoration options. We address these issues in the context of an analysis of natural and artificial diversions, crevasse splays, and small sub-delta lobes. We suggest that episodic large diversions and crevasses (>5000 m 3 s −1 ) can build land quickly while having transient impacts on the estuarine system. Small diversions (<200 m 3 s −1 ) that are more or less continuously operated build land slowly and can lead to over-freshening and water level stress. We use land building rates for different sized diversions and impacts of large periodic inputs of river water to coastal systems in the Mississippi delta to conclude that high discharge diversions operated episodically will lead to rapid coastal restoration and alleviate concerns about diversions. Single diversion events have deposited sediments up to 40 cm in depth over areas up to 130–180 km 2 . This approach should have broad applicability to deltas globally. [ABSTRACT FROM AUTHOR]

Abstract: The impact of diverted Mississippi River water on temperature, salinity, total suspended sediment (TSS) and chlorophyll a were monitored in the Breton Sound estuary from September 7, 2000, to August 28, 2002. Twenty-seven transects were carried out using a flow-through system to continuously measure temperature, salinity, turbidity and fluorescence, and discrete water samples were taken at 16 locations for calibration. Discharge from the river diversion structure ranged from 0 to 213m3/s, with several large pulses of water released during the spring of 2001 and 2002. There was a strong seasonal temperature signal in the estuary, with summer highs of >30°C and winter lows of <12°C. Incoming river water was usually cooler than estuarine waters, with temperatures as low as 6°C, but generally equilibrated to the rest of the estuary within several kilometers. Salinity in the upper estuary was fresh throughout the study, and increased up to 14 and 30PSU along Western and Eastern routes, respectfully, with distance from the diversion. Discharge from the river diversion greatly affected salinity, with the large spring pulses often causing the entire estuary to become fresh for a short period of time (<1month). There was also a temporal lag of about two weeks between discharge and salinity in the lower estuary. River water entering the estuary had TSS concentrations ranging from 40 to 252mg/L, with an average of 118mg/L. Sediment from the river diversion reached about 10–15km into the estuary during spring pulses. There were highly fluctuating TSS concentrations at the Gulf end of the estuary during winter and spring, due to wind resuspension. Chlorophyll a concentrations were highest in mid-estuary during summer and fall low discharge, and lowest during winter and spring high discharge. Chlorophyll a levels were generally less than 10μg/L in the upper estuary, with concentrations rising in the mid-estuary generally to 20–30μg/L during late summer and fall, and decreasing in the lower estuary. There were several periods of less than a month with unusually high chlorophyll a concentrations, ranging from 38 to >60μg/L, that occurred during late summer and early winter. [Copyright &y& Elsevier]

During the spring of 2001, we monitored nutrient concentrations and stoichiometry of diverted Mississippi River water as it flowed through the Breton Sound estuary, Louisiana, USA. River water was discharged through a diversion structure at Caernarvon as a two-week pulse that peaked at 220 m3 s−1. There were reductions in observed concentrations of TN, TP, DIN, DIP and DSi, of up to 44%, 62%, 57%, 23%, and 38%, respectively, as water flowed through the estuary. TN, TP, DIN, DIP and DSi concentrations in the river were 137–140, 5.0–5.1, 104–153, 1.1–1.3 and 114–121 μM, respectively, and 36–122, 1.8–3.6, 13–119, 0.3–1.8 and 29–110 μM, respectively, at the Gulfward end member stations. The DSi:DIN ratio rose from 0.9 at the Caernarvon diversion to 2.6 at the Gulf end member station, while the DIN:DIP ratio fell from 107 to 26. This study shows that freshwater diversions can significantly alter riverine nutrient concentrations and ratios and reduce the overall amount of exported nitrogen. [Copyright &y& Elsevier]

Canals and spoil banks have contributed significantly to high rates of wetland loss in the Mississippi Delta. There has been relatively little research on management of canals and spoil banks and this needs to be a significant component of restoration of the delta. We analyze research on the role of backfilling canals in the context of delta restoration with special reference to Turner and McClenachan (2018) who state that if all canals were backfilled, it could significantly reduce or even reverse wetland loss and that most wetland loss is caused by canals. We agree with Turner and McClenachan that canals have been a significant cause of wetland loss in the Mississippi Delta and that removing spoil banks and backfilling canals should be an integral part of delta restoration. However, a number of factors need to be considered when choosing which canals to backfill including possible enhanced erosion due to exposure to wave action for newly created and remnant marsh, the current and future production history of oil and natural gas wells associated with canals, and other restoration activities in oil and gas fields. Turner and McClenachan's analysis using wetland loss patterns in 15-min quadrangles suggesting that canal density can explain most wetland loss in coastal Louisiana is flawed because of scale problems and other impacts of oil and gas activity. These impacts include subsurface induced subsidence and the impact of produced water and toxins on wetlands that are largely unrelated to surface alteration due to canals and spoil banks. [ABSTRACT FROM AUTHOR]

This review synthesizes the knowledge regarding the environmental forces affecting water level variability in the coastal waters of the Mississippi River delta and relates these fluctuations to planned river diversions. Water level fluctuations vary significantly across temporal and spatial scales, and are subject to influences from river flow, tides, vegetation, atmospheric forcing, climate change, and anthropogenic activities. Human impacts have strongly affected water level variability in the Mississippi River delta and other deltas worldwide. Collectively, the research reviewed in this article is important for enhancing environmental, economic, and social resilience and sustainability by assessing, mitigating, and adapting to geophysical changes that will cascade to societal systems in the coming decades in the economically and environmentally important Mississippi River delta. Specifically, this information provides a context within which to evaluate the impacts of diversions on the hydrology of the Mississippi delta and creates a benchmark for the evaluation of the impact of water level fluctuations on coastal restoration projects worldwide. [ABSTRACT FROM AUTHOR]

The objectives of this study were to examine changes in accretion and elevation change over periods of up to 15 years for the Biloxi marsh complex (BMC) in southeastern Louisiana, part of the Mississippi Deltaic Plain, identify factors affecting accretionary dynamics, and put these findings in the context of ongoing restoration. We present elevation and accretion data from Surface Elevation Table (SET) and feldspar marker horizon sites first established in 2003. The sites were clustered in two areas (East and West) in the central BMC on the eastern edge of the Mississippi delta. Accretion markers were used in conjunction with elevation measurements to calculate shallow subsidence. These data were analyzed along with similar data from nearby Coastwide Reference Monitoring System (CRMS) sites located around the periphery of the BMC. Elevation decreased at the Western sites by −0.35 ± 0.13 cm/yr, and increased at the Eastern and CRMS sites by 0.40 ± 0.03 cm/yr and 0.72 ± 0.09 cm/yr, respectively. The rate of accretion was similar at the Western (0.49 ± 0.14 cm/yr) and Eastern (0.64 ± 0.07 cm/yr) sites, and over twice as much (1.30 ± 0.11 cm/yr) at the CRMS sites. Shallow subsidence, calculated as the difference between vertical accretion and surface elevation change, was 0.76 ± 0.49 cm/yr at the Western sites, 0.23 ± 0.06 cm/yr at the Eastern sites, and 0.58 ± 0.11 cm/yr at the CRMS sites. These trends are consistent with the observation that sediment is brought in from Chandeleur Sound to the east and is attenuated as deposition occurs across the landscape from east to west, and that levee flank depressions associated with Bayou La Loutre, an abandoned Mississippi River distributary ridge, are causing locally high subsidence in the Western region. Without intervention, these localized areas of the Western region will be submerged within the next several decades at current rates of elevation loss and eustatic sea-level rise, while the Eastern sites and the wetlands on the periphery of the BMC are likely to keep pace with sea level rise well into the second half of this century. These results demonstrate the importance of accurate knowledge of both subsidence and accretionary dynamics in determining coastal wetland sustainability and restoration approaches. • There was elevation gain at the Eastern and CRMS sites and elevation loss at the Western sites of the Biloxi Marsh complex. • The Western region will most likely be submerged within the next several decades without intervention. • The Eastern region and the wetlands on the periphery of the BMC will most likely keep pace with sea level rise. • Sediment is brought in from Chandeleur Sound to the east and deposited across the landscape from east to west. • Levee flank depressions associated with Bayou La Loutre are causing locally high subsidence in the Western region. [ABSTRACT FROM AUTHOR]

One of the proposed methods for restoring the disappearing Mississippi Delta is sediment diversion which uses channels and structures to divert water and sediment from the Mississippi and Atchafalaya Rivers into adjacent basins. This study presents a comprehensive review of geological and physical aspects of sediment dynamics in the Mississippi River Deltaic Plain (MRDP), with special reference to diversion studies over the past two decades. We synthesize these studies, present the current understanding of sediment diversions in the context of sediment dynamics, identify multiple key knowledge gaps, and make recommendations for future studies. To maximize net land building in the MRDP, management strategies should be focused on (a) enhancing river sediment delivery (both mud and sand), (b) increasing sediment retention in receiving basins and (c) minimizing erosion in bays and estuaries. Compared with extensive studies of land building, there have been relatively fewer studies of erosional processes. A heterogeneous coastal geological framework, cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the MRDP. Sediment retention rates are highly sensitive to spatial and temporal scales, types of sediments and delivery season. Sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes. Structures and devices that improve sediment retention, trap sediments, dissipate waves, and build living shorelines should be explored and cost-to-benefit analysis is needed. Long-term planning should consider more landward diversions, strategic community relocation, and nonlinear response of the complex sedimentary system of the MRDP. • A review and synthesis of diversion studies and sediment dynamics in the Mississippi River Deltaic Plain. • Explore the methods to maximize land building by enhancing sediment delivery, increasing retention, and minimizing erosion. • Present current understanding, identify knowledge gaps and recommend future work for sediment diversions. [ABSTRACT FROM AUTHOR]

Coastal Louisiana is dominated by the Mississippi River Deltaic Plain, which is composed of a series of overlapping and truncated sub-delta lobes that formed an extensive coastal wetland – open water shallow bay and low relief upland mosaic across the entire coastal zone (∼25,000 km2). These coastal wetlands have been eroding at an alarming rate during the past century, coincident with major modifications to the deltaic landscape. One of these former modifications included extensive leveeing of the Mississippi River, essentially isolating the river from the coastal basins. One restoration technique involves re-connecting the previously isolated coastal basins to the river via diversions, which would once again allow the influx of freshwater, sediments and nutrients into the coastal basins. We review issues on potential impacts of nutrient loading from the river as a consequence of river diversions, focused on nitrogen (N) and phosphorus (P) dynamics including denitrification related to carbon dynamics. We also examined how water residence times in these shallow systems, compared to deeper, often seasonally stratified, coastal systems, are more likely to lead to greater N removal potential in the context of a large scale river diversion helping to reduce N transport to the coastal ocean. [ABSTRACT FROM AUTHOR]

Abstract River systems worldwide have become substantially influenced by human activities, including land use changes, river diversion operations, and flood control measures. Some of the unambiguous and best studied examples of effects of enhanced eutrophication on biotic resources can be found in Louisiana estuaries at the terminus of the Mississippi-Atchafalaya River system. The Mississippi River delta has experienced large losses of coastal wetlands due to a combination of human impacts and sea-level rise. State and Federal agencies are moving ahead with plans for building large-scale river sediment diversions, which will capture maximum sediment during spring flood pulses and direct a sediment subsidy into the eroding coastal basins. These large-scale river sediment diversions will also substantially increase freshwater and nutrient inputs and are likely to affect algal bloom formation, including harmful cyanobacterial blooms. There are concerns that discharge of river water containing high concentrations of N, P and Si may trigger algal blooms in the coastal receiving basins. River sediment diversions, as any other flood pulsing, will likely be disruptive to the coastal ecology and so balancing the benefits of slowing coastal land loss against potential negative effects on water quality remains a formidable management challenge. We review here the physical, chemical and biological factors affecting primary production in shallow coastal systems and provide known data on ecosystem response to freshwater diversions, large and small. We also discuss potential management approaches to mitigate the negative impacts of the diversions on the health and stability of the coastal food webs. [ABSTRACT FROM AUTHOR]

Abstract: Pulsed re-introduction of Mississippi River water into the deltaic plain has been proposed as a wetland restoration strategy for coastal Louisiana. In this study, the hydrodynamic response of the Breton Sound estuary to a two-week pulse of Mississippi River water via the Caernarvon river diversion structure was investigated using a barotropic, three-dimensional, Finite-Volume Coastal Ocean Model (FVCOM). The numerical model was driven by tidal and subtidal forcing at the open Gulf boundary, freshwater discharge from the Caernarvon river diversion structure, as well as wind stress at the water surface. After successfully validating the model with field observations, three numerical experiments were run to assess the response of current, water level, and marsh flooding to different diversion discharge scenarios. The three scenarios considered were: a pulsed scenario of ∼200 m3 s−1 corresponding to the actual diversion discharge in March 2001, a constant discharge scenario of 40 m3 s−1 corresponding to the annually averaged discharge of 2001, and a scenario with no discharge. Numerical simulation results indicated that constant 40 m3 s−1 discharge caused little change in wetland inundation comparing to the no discharge case and, thus, inter-exchange between deep channels and the wetlands was not improved by this rate of diversion discharge. In contrast, the two-week ∼200 m3 s−1 discharge caused enhanced water exchange between wetlands and adjacent water bodies, substantially increasing water velocity in the bayous and channels of the upper estuary. These effects occurred in the estuary to about 20–25 km from the diversion structure, and caused a noticeable increase in down-estuary residual current with a significant reduction of local estuarine residence times for the whole estuary. Beyond 30 km from the diversion structure, the impact of high water discharge was small and the hydrodynamics was mostly controlled by tides and wind. [Copyright &y& Elsevier]

This paper presents a comprehensive review of shear strength measurements in wetland soils, which can be used to make inferences of the influence of nutrients and sediments on wetland health. Ecosystem restoration is increasing across the Gulf of Mexico and in other coastal systems, with management questions related to soil strength among the most critical to address for the sustainability of restoration programs. An overview of geotechnical engineering principles is provided as a starting point to understand basic soil mechanics concepts of stress, effective stress, pore-water pressure, unit weight, and shear strength. The review of wetland shear strength measurements focuses on the hand-held vane shear, torvane, cone penetrometer, and wetland soil strength tester. This synthesis shows that vane shear measurements can identify the shear strength trend in horizontal and vertical spaces and may be an indicator of wetland soil strength. However, the significant uncertainty of the vane shear measurements may preclude making conclusions about shear strength values without further testing and calibration of the devices. The torvane results show considerable scatter such that it is not recommended for quantitative shear strength measurements. The cone penetrometer represents a technique that is independent of operators and provides a high density of measurements with depth. It signifies the state-of-practice of wetland shear strength testing and is a reasonable tool to measure spatial and temporal variations in soil strength and other geotechnical properties (e.g., pore-water pressure, soil moisture, resistivity, and temperature) in wetlands. The wetland soil strength tester provides insight into the wetland soil resistance in the first 15 cm, which is the zone where most belowground biomass is present. Recommended future research includes evaluating the uncertainty in all in-situ soil strength testing methods, developing relationships between different field instruments, and establishing consistent statistical methods and field-testing procedures to make inferences and assessments. • A thorough review of shear strength measurements in wetlands is presented. • Studies using the torvane, hand-held vane, cone penetrometer, and wetland shear strength tester are discussed. • Review indicates a need for consistent insitu methodology to assess shear strength over a range of vegetation and conditions. [ABSTRACT FROM AUTHOR]