The Lake Pontchartrain Basin in southeast Louisiana is a 9645 mi² (24,980 km²) area composed of low-lying coastal forests hydrologically coupled with deltaic estuaries associated with the Mississippi River and the Gulf of Mexico. The two largest cities in Louisiana, New Orleans and Baton Rouge, are within the basin. Since European settlement in 1718, population increase, landscape modifications, accidental events, and exploitation of natural resources have had a profound impact on the natural landscape.

A database of several hundred environmental impacts from 1718 to 2002 was developed. Examples of impacts are introduction of invasive species, extirpation of species, deforestation, dredging of wetlands, land clearing, Mississippi River modifications, oil and gas activities, the development of superfund (contamination) sites, and water pollution. The chronology was evaluated four ways to identify the most significant ecologic impacts. Based on the analyses of the chronology, five periods of anthropogenic activity are proposed to have resulted in the most significant negative impacts to the Lake Pontchartrain Basin since European settlement in 1718. These five periods are:

These selected activities affected approximately 76% of the area of the basin and may have reduced habitat quality by 50% initially, which later recovered to nearly 60% functionality. Generally, these impacts still result in latent impairment or increasing damage. Three of the periods contribute significantly to the wetland land-loss crisis in southeast Louisiana.

The near 300-year legacy represents unsustainable land or natural resource utilization. Many of the impacts predate modern physical and biological techniques, and, therefore, a true baseline of conditions without alteration by European settlers is poorly known for the Lake Pontchartrain Basin.

The Pontchartrain Basin extends over 44,000 km² from northern Mississippi to the Gulf of Mexico and includes one of the largest and most important estuarine systems in the United States. The basin supports a variety of environments, from woodlands in the north to wetlands in the south, and a growing socioeconomic infrastructure that has led to rapid development of the southern half of the basin over the past two centuries. To properly administer this infrastructure, managers need to understand the complex geologic framework of the basin and how it will respond to continued sea-level rise, variable rates and magnitudes of land subsidence, and human alteration of the landscape. This article summarizes the body of work that describes the regional evolution and stratigraphic architecture of the Pontchartrain Basin.

The northern two-thirds of the basin is underlain by a stratigraphy of undifferentiated sands and clays deposited throughout the Plio-Pleistocene by glacially influenced rivers. These deposits were weathered and incised by rivers during sea-level low stands, forming a series of terraces that increase with age from south to north. The southern third of the basin is composed of estuaries formed during the Holocene, while shoreline processes created a series of sandy barriers that restricted communication to the Gulf of Mexico. The Mississippi River completed the geologic development of the basin by building a sequence of subdelta lobes along this southern margin over the past 5000 years, further sealing it from the open Gulf of Mexico. Presently, the modern Mississippi River bypasses the estuarine environment and only contributes sediments during flood events when the river overtops the levee system. Sea-level rise, subsidence within the Holocene delta-plain deposits, and movement along numerous fault systems are the active natural processes that continue to affect basin geomorphology.

Very high subsidence rates are routinely documented within the Pontchartrain Basin and coastal Louisiana. Subsidence promotes land loss and degrades the integrity of infrastructure and ecosystem health. Despite its vast impact on the regional landscape, the precise causes of the subsidence are not well understood; contemporary research attributes measurements of subsidence to many different processes. Because individual subsidence studies often focus on a singular subsidence process and use alternative methodologies, results may not be comparable with or complementary to the results of other studies, hindering attempts to synthesize the collective body of research into a comprehensive regional understanding. This article presents a review of contemporary subsidence research to define the most influential processes in coastal Louisiana. The processes are grouped into six categories: tectonics, Holocene sediment compaction, sediment loading, glacial isostatic adjustment, anthropogenic fluid withdrawal, and surface water drainage and management. Each process category is discussed in a uniform context designed to indentify comparable characteristics and the relative spatial and temporal scales in which they occur. Establishing the full range of influential subsidence processes and providing a means of comparison is a first key step in synthesizing a comprehensive understanding of subsidence in coastal Louisiana.

Describing the nature and extent of land resources and changes over time has become increasingly important, especially in rapidly growing metropolitan areas. In this study, two Landsat satellite image scenes were examined to identify land use and land cover changes for the Lake Pontchartrain Basin between 1982 and 2005. Classification accuracies were based upon ground truth data obtained by global positioning system field collection and photo interpretations. A postclassification change detection analysis was used to identify areas that have experienced conversions in land use or land cover. Comparisons of the land cover maps reveal that a steady growth in population and an increase in commercial and residential development have caused extensive changes to critical habitats throughout the basin. The maps also indicate that the loss of coastal wetlands, combined with shoreline erosion, remains one of the most serious environmental problems facing the Pontchartrain Basin today. The postclassification change detection analysis showed that critical habitats accounted for nearly 40% of the total urban growth between 1982 and 2005. Results also showed that for the time period studied, approximately 25 square miles (15,994 acres) of marsh was converted to open water. This is an overall average decrease of approximately 640 acres per year.

The geomorphology and processes of land loss in the Pontchartrain Basin for the period 1990–2001 are mapped and the results of the classification presented. This data set is compared to an earlier classification, which covered the period 1932–1990 to determine the status and trends of wetland loss in the Pontchartrain Basin between 1932 and 2001. Results of the land loss classification indicate land loss rates have remained steady between the two periods (13 km²/y between 1932 and 1990 and 12 km²/y between 1990 and 2001). The geomorphology and processes of loss have shifted from primarily interior loss as a result of direct removal and submergence in the first period to primarily shoreline loss as a result of erosion in the second period.

Three areas of concentrated land loss between 1990 and 2001 represent portions of the upper, middle, and lower basin. Land loss at all three locations shifts from interior to shoreline loss between the two periods. In the second period, interior loss through the process of submergence is still occurring but at much reduced rates at locations 1 and 2. Interior loss through the process of direct removal is only occurring at location 1 during the second period and accounts for less that 2% of the loss. At location 3, land loss is completely in the form of shoreline erosion during the second period.

Numerical experiments of multiple freshwater diversions into the Pontchartrain Estuary under tidal forcing were conducted to evaluate the impact on salinity and tidal flow distribution. A validated numerical hydrodynamic and transport model was used to assess the impacts on tidal flows, circulation, and salinity as a function of additional freshwater input in the estuary from hypothetical diversions combined with channel modifications in the Mississippi River Gulf Outlet. The cumulative and specific impacts were compared with existing conditions. It was concluded that upper and middle estuarine salinity regimes are coupled, and diversion flows need to be managed in accordance with historic inputs. This study also showed that if total freshwater input is not of an order similar to the existing natural tributary flow, the average salinity in the upper estuary could be reduced by 1.5 ppt (±0.5 ppt), which is approximately 40% of the existing long-term salinity of the upper estuary. The additional flow into the upper estuary will produce changes in the flow through the tidal passes on the order of 5%–6%, will decrease hydraulic detention times in the estuary, and will cause an additional increase in the ebb-dominance of the estuary.

Algal blooms have been recorded in the Pontchartrain Estuary when elevated nutrient loads combine with other ambient conditions, e.g., low salinity and low turbidity. Nutrient and sediment loads were quantified and incorporated into a dynamic mass balance model for the upper Pontchartrain Estuary to determine the timing of conditions that can lead to algal blooms. General nutrient and sediment loading relationships were developed for the tributaries of the Pontchartrain Estuary where data were available; these relationships were extended to estimate the loading for ungauged areas. The annual water yield for the drainage basin was found to be 500 mm. The annual nutrient loadings from all sources were found to be 21,000 t (13 g m⁻² y⁻¹) of total nitrogen and 2700 t (1.6 g m⁻² y⁻¹) of phosphorus for Lakes Maurepas, Pontchartrain, and Borgne. The basin sediment yield was calculated to be 42 t km⁻². Nutrient and sediment concentrations in the Mississippi River were combined with the estimated leakage through the Bonnet Carré Spillway to obtain nutrient loads related to the river stage. The mass-balance model was applied to assess the occurrence of algal blooms for the period 1990–2008; the model predicted all five observed algal blooms but also indicated a potential for two more that were not documented. The strongest potential for algal blooms was in the northwest quadrant of Lake Pontchartrain although the highest dissolved inorganic nitrogen concentrations were predicted for the southwest quadrant in connection with spillage from the Bonnet Carré Spillway. The lower than expected response of the southwest quadrant may be associated with the higher turbidity due to the Spillway plume.

La afloración de algas ha sido observada en el Estuario Pontchartrain cuando las cargas elevadas de nutrientes se combinan con otras condiciones ambientales (e.g., baja salinidad y baja turbiedad). Las cargas de nutrientes y de sedimentos fueron cuantificadas e incorporadas en un modelo dinámico de balance de masa en la region superior del Estuario Pontchartrain para determinar el momento en que las condiciones son propicias para la afloración de algas. Para los casos donde habían datos disponibles, se establecieron relaciones entre los nutrientes generales y las cargas de sedimentos de los tributarios del Estuario Pontchartrain; en las áreas donde no hubo datos disponibles, estas relaciones fueron utilizadas para estimar las cargas. El rendimiento de agua anual para la cuenca hidrográfica fue de 500 mm. Las cargas anuales de nutrientes de todas las fuentes fue de 21,000 toneladas de nitrógeno total y 2700 toneladas de fósforo. El rendimiento de sedimentos fue calculado en 42 toneladas/km². Las concentraciones de nutrientes y sedimentos en el Río Mississippi fueron combinadas con las de las estimadas por la fuga de agua por Vertedero Bonnet Carré para obtener cargas de nutrientes relacionadas con el nivel del Río. El modelo de balance de masa fue aplicado para evaluar la ocurrencia de la afloración de algas para el periodo entre 1990 y 2008. Este modelo predijo las 5 afloraciones de alga observadas y también indicó el potencial para 2 afloraciones más que no fueron documentadas. El área con el mayor potencial al desarrollo de afloración de algas fue el cuadrante noroeste del Lago Pontchartrain, a pesar de que las mayores concentraciones de nitrógeno inorgánico disuelto fueron predichas para el cuadrante suroeste en relación con la descarga del Vertedero Bonnet Carré. La respuesta, más baja de la esperada, en el cuadrante sureoeste puede estar asociada con mayores niveles de turbiedad debido a la mancha de la descarga del vertedero.

Studies of Lake Pontchartrain benthic invertebrates and submersed aquatic vegetation (SAV) from 1996 through 2005 provided the opportunity to determine community responses to severe disturbances including: recovery from shell dredging; effects of anoxia and hypoxia from saltwater intrusion; the 1997 Bonnet Carré Spillway opening; a prolonged drought resulting from an El Niño Southern Oscillation; and effects of Hurricane Katrina. We included reviews and updates of our recently published work, an integrated analysis of findings, and restoration recommendations. During this decadal study there were no prolonged periods of “normal” conditions. Instead, benthos and SAV were experiencing or recovering from significant temporal disturbances. This made it difficult to use the abundance of a particular set of organisms at a point in time to evaluate habitat quality or restoration success without considering past disturbance effects. Benthos species diversity and the relative abundance of stress tolerant groups such as annelids were good indicators of short-term adverse conditions, but rapid changes occurred in response to salinity and dissolved oxygen. Rangia cuneata was a good indicator of the extent of hypoxia and long-term damage from hurricanes. The distribution and abundance of SAV decreased with turbidity and nutrient increases, and Vallisneria americana and other freshwater SAV decreased with prolonged salinity increases. Resistance to and resilience after disturbances and natural changes during long term cycles have to be considered in evaluating habitat condition and restoration success.

The estuarine ecosystems of southeastern Louisiana are threatened by numerous environmental impacts such as wetland loss, coastal development, and overharvesting of natural resources. If the relative health of different estuaries can be determined, then management efforts might be focused on those regions needing the most protection. Unfortunately, estuaries are by definition dynamic, precluding easy comparisons of relative environmental health. Meta-analyses can be used to overcome problems associated with this natural variability. Analyzing sizable ecological data sets that cover large spatial and temporal scales is helpful in assessing relative ecosystem health among different regions. To compare the health of four estuarine regions of southeastern Louisiana (the Barataria Basin, Lake Maurepas, Lake Pontchartrain, and the Biloxi Marsh and Chandeleur Islands region), we calculated taxonomic distinctness and variation in taxonomic distinctness for fishery-independent data collected from three habitats: demersal, nearshore, and pelagic habitats. Taxonomic distinctness is a biodiversity index that measures taxonomic distance between species collected in a single sample. This taxonomic-based method is robust to differences in sample size and generally more useful for large-scale meta-analyses than other diversity measures. We analyzed data collected by trawls (demersal habitats), beach seines (nearshore habitats), and gill nets (pelagic habitats) over various periods in the last half century. Demersal fish assemblages from Lake Pontchartrain and pelagic fish assemblages from the Barataria Basin were more affected than fishes collected in similar habitats in the other regions. Nearshore fish assemblages, though, were equally healthy across all regions studied.

Lake Pontchartrain in southeastern Louisiana is the largest of several shallow estuaries that together cover over 15,000 km². Wetlands, forests, and large urban areas surround the lake. Primary transport mechanisms of sediments to Lake Pontchartrain include urban runoff, major diversions of the Mississippi River, discharge from streams along the north and west shores, and tidal circulation. Sediments deposited in Lake Pontchartrain are subjected to resuspension and mixing by natural and human activities. Bioturbation and water turbulence throughout the lake are the major mixing agents, and mechanical shell dredging has reworked much of the lake bottom over the last century. Sediment characterization through direct sampling and geophysical surveys indicates that these processes continually rework the top meter of sediment.

The lake receives discharge from roadways and industrial and agricultural sources. Contaminants from these sources accumulate in the lake sediments and are an important contributor to the degradation of the estuary. Decline in populations of various benthic organisms, such as shrimp and clams, has been documented in the lake. To characterize the health of this important estuary, the U.S. Geological Survey (USGS) conducted a comprehensive evaluation of the geology, geomorphology, coastal processes, and environmental condition of the Pontchartrain Basin from 1994 to 1997. This report presents an assessment of sediment distribution and quality using a multidisciplinary approach to characterize the influence of various physical and chemical parameters: nearsurface stratigraphy, major trace metal concentrations (Cu, Pb, Zn, and Ni), and short-lived radionuclides (²¹⁰Pb, ⁷Be, and ¹³⁷Cs). The results are compared with water-circulation patterns to determine high-resolution sedimentation patterns in the lake. The data show a significant increase in trace metals in the top 1 m of lake sediments. Above this horizon, pollen analysis indicates a correlation with land clearing in the area, a proxy for increasing human development of the surrounding landscape and an increase in surface run-off. The data also show that the top meter of sediment undergoes frequent resuspension during high-energy circulation events and via circulation gyres in the lake. This regular turnover does not allow stratification of recently deposited sediments, restricting the sequestration of contaminated material that enters the lake.

The Bonnet Carré Spillway is a flood-control structure that diverts Mississippi River water into Lake Pontchartrain during exceptionally high river stages. Because of elevated water levels in the Mississippi River in the spring of 1997, the Bonnet Carré Spillway was opened on March 17 and fully closed on April 18. The total volume of water discharged into Lake Pontchartrain was approximately 11.8 km³, or two times the volume of the lake, and the total mass of sediment discharged into the lake was approximately 7.1 × 10⁸ kg (780,000 US tons). In 1996, 757 surface sediment samples were collected in Lake Pontchartrain and were analyzed by x-ray fluorescence spectroscopy for major cation constituents. These same sites were revisited following the 1997 Mississippi River discharge event. Analysis of the 1996 and 1997 lake-bed sediment samples was accomplished utilizing fundamental statistical and graphical methods. Element concentration contour maps and variograms for the major cations illustrate meaningful differences between the pre- and postspillway sediment samples that are not readily apparent in the analysis of the descriptive statistics alone. Major cations exhibited significantly greater spatial continuity in the postspillway samples relative to the preceding year. The concentrations of aluminum and silicon in the postspillway sediments are considered to reflect, respectively, relative variations in clay and silt contribution to total sediment. The higher concentrations of magnesium in samples collected prior to the river diversion represent adsorption of magnesium onto exchange sites in surface sediments due to exposure to more saline waters.

In recent years, the focus on issues regarding water quality has shifted from a local site-specific view of a problem toward a holistic view where the health and hydrologic functionality of entire watersheds are of concern. One measure of watershed health is fluvial geomorphology. The goal of this project was to map the morphology of the streams of the Amite watershed of the Pontchartrain Basin using a reach sensitivity index (RSI). The classification scale is based on stream geomorphology, hydrology, and ecological characteristics, where rank is given based on morphological complexity and associated wetlands. The RSI classification method used data from 18 field sites along with digital orthophoto quarter quadrangle 1-m resolution infrared aerial imagery, gap analysis program land cover data, and digital elevation models generated from light detection and ranging. In addition to mapping, issues related to watershed impairment and restoration were identified. Key anthropogenic impacts observed include sand and gravel mining, stream manipulation, profuse litter, and overall water quality. The watershed in this study is proximal to urban areas associated with New Orleans and Baton Rouge, Louisiana, so another theme, waterfront development, was also identified as a source of stream impairment. Reach sensitivity index stream classification can serve as a tool for conservation strategies and watershed restoration, where the ultimate goal is to restore natural hydrological and ecological functionality to impaired streams. Maps generated in the RSI project can further aid in regional planning, without which, rapid urban development and continued stream manipulation within the Amite and neighboring stream systems will result in accelerated watershed impairment and water-quality degradation.

In general, many of the swamps of coastal Louisiana, U.S.A., are highly degraded, and several are converting to marsh and open water. The initial purpose of this study was to determine the feasibility, and potential benefits, of reintroducing waters of the Mississippi River into the degraded Maurepas swamp, located in the Pontchartrain Basin of southeastern Louisiana. Early in the year 2000, 20 sites were selected in three different habitat types characterized by moving fresh water (throughput sites), stagnant, nearly permanently flooded (relict sites), and areas prone to saltwater intrusion events (degraded sites). Paired 625-m² plots were outfitted with litter-fall traps, herbaceous subplots, and wells for measuring interstitial soil salinity. From 2000–2006, diameter growth was followed for 2219 trees, and herbaceous production was estimated using mid- and late-growing season clip plots. Overall, primary production was dominated by trees early in the study, but switched to herbaceous vegetation as parts of the ecosystem converted from swamp to marsh. Salt stress was the primary cause of tree mortality in areas of low density, whereas stagnant standing water and nutrient deprivation appear to be the largest stressors at interior (relict) sites. The 2005 hurricanes caused wind throw of up to 100% of midstory trees in areas of low canopy density and was negligible when basal areas of baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica) were greater than 30 m² ha⁻¹. Using spectral signatures of the 625-m² plots, the aerial extent of habitat types revealed that the vast majority of the Maurepas swamp is either relict or degraded. Without a river reintroduction in the near future, as well as harnessing other point and nonpoint sources of fresh water, the Maurepas swamp will continue its clear trajectory to marsh and open water.

Our scientific understanding of the marshes along the north shore of Lake Pontchartrain, Louisiana, is limited in terms of the processes required to sustain them and how to best manage them in the face of predicted rising sea levels. Subject to localized subsidence and urban development, these marshes may also be affected by increased nutrient loading in the future from proposed Mississippi River diversions and continued urbanization. This study presents data on marsh surface elevation change across a series of experimental plots located in Big Branch Marsh National Wildlife Refuge, Louisiana, that were subject to varying additions of phosphorus and nitrogen as well as a lethal herbicide treatment. These plots were also affected by Hurricanes Katrina and Rita in 2005. The rate of marsh elevation change prior to the storm suggests these marshes were maintaining elevation in the face of sea-level rise. A dramatic increase in elevation occurred following the storms but was followed by a proportional decrease in elevation. Soil data indicate the increase was caused by an influx of highly organic material at all plots. The results show how both storm and nonstorm processes contribute to elevation change and the maintenance of these marshes in the face of sea-level rise.

Aboveground plant community dynamics in an oligohaline marsh at Big Branch Marsh National Wildlife Refuge, Louisiana, U.S.A., were assessed in response to nutrient loading (3 N × 3 P factorial) and disturbance (both planned herbicide treatment and stochastic tropical storm or hurricane disturbances). Sampling was conducted seasonally from April 2004 to September 2006. Spartina patens and Schoenoplectus americanus are codominant plant species in this marsh. Although Spartina patens displayed increased aboveground cover under the low N addition (20 g N m⁻² y⁻¹) relative to ambient conditions or high N addition (40 g N m⁻² y⁻¹), increased N or P loading did not result in a shift in plant community composition or species richness during the study period. Schoenoplectus americanus consistently had higher leaf tissue N and generally higher leaf tissue P than Spartina patens regardless of treatment. Our results indicate that Schoenoplectus americanus is more resilient than Spartina patens to disturbances that do not increase marsh surface elevation, such as minor disturbances (e.g., prolonged flooding events) or prescribed burning, which is often utilized as a management technique to increase the relative abundance of Schoenoplectus americanus in this marsh type. Similarly, Schoenoplectus americanus was able to recolonize the herbicide treatment plots to some degree during the study via a combination of seed bank and rhizome tillering, whereas Spartina patens remained essentially absent. Hurricane Katrina deposited significant amounts of sediment (average of 27 cm) into plots that survived the storm (August 29, 2005). By 2006, this elevation increase resulted in a significant increase of both Spartina patens cover and species richness, suggesting that a shift in the relative abundance of the two codominants is mitigated by disturbance type and the resultant effect of disturbance on the abiotic environment, particularly marsh surface elevation.

Coastal Louisiana has entered a period when the convergence of at least two powerful processes is working against its survival. Meteorological processes driving tropical systems have more frequently generated category 4 and 5 hurricanes and, although not a certainty, more destructive hurricanes are predicted for coming decades. Since the 1950s, the processes driving coastal land loss in Louisiana have continued only slightly abated. A deteriorating coast and historically inadequate levees make Louisiana acutely susceptible to the negative consequences of hurricane surge events. Coastal Louisiana's crisis of vulnerability is the continued temporal overlap of weakened hurricane protection with more frequent, intense hurricanes.

The multiple lines of defense strategy proposes that two essential elements of the Louisiana coast be managed and perpetuated, which together can economically sustain the coast. The two planning elements are (1) using natural and manmade features (lines of defense) that directly impede storm surge or reduce storm damage and (2) establishing and sustaining habitat goals. The strategy can be broadly considered as integration of structural and nonstructural flood protection with coastal restoration. Extensive storm surge modeling by the U.S. Army Corps of Engineers has verified the presence of “critical landscape features,” which are existing lines of defense that beneficially impede storm surge. The proposed strategy is potentially a unifying vision for the coast: embracing environmental habitat restoration as well as engineered flood protection. Application of the strategy is more consistent with traditional land use than modern land use in coastal Louisiana.

The multiple lines of defense strategy goals can be articulated through maps and tested with hydrodynamic and habitat models. The paramount product of this planning is a single map indicating the desired future elements of the coast that provide essential ecologic services and an adequate regional flood system to perpetuate the economy of the region.

There have been many plans developed by governmental agencies to restore the coastal landscape in the Pontchartrain Basin. This article traces the development of coastal restoration planning in Louisiana and highlights the elements of three state and federal plans that have been developed in the last decade. While some specifics have changed, the fundamental approaches on which the plans are based largely remain the same: diversions of Mississippi River water and sediment, marsh creation using material dredged from another location, and shoreline protection. All plans accept that change is inevitable at the coast but more recent plans are less constrained by the impacts of these changes on local populations. Rather they anticipate adjustment and change within communities as well. Questions remain as to whether these measures will be enough in the face of the great uncertainty the twenty-first century holds and how science will be applied to support implementation.

It is generally felt in the water resources community that the most significant twenty-first century public works projects will be those undertaken to correct environmental damage caused by twentieth century projects. A second axiom is that the switch from economic development to restoration and mitigation, what we call redemption, often will be precipitated by disaster. Finally, it must be expected that the repair project will cost far more than the initial public investment but also may have economic revitalization potential far exceeding anticipated environmental benefits. We examine this cycle for the federally funded Mississippi River Gulf Outlet (MRGO) navigation project east of New Orleans, beginning with its much heralded birth in 1963 as a 122 km long free-flowing tidal canal connecting New Orleans to the Gulf of Mexico and ending with its recent de-authorization and closure. We track the direct and indirect effects of the project through its commercial failure, and then on to the official denial, the pervasive environmental impacts, and finally exposure of its role in flooding New Orleans during Hurricane Betsy in 1965 and more seriously during Hurricane Katrina in 2005. Post de-authorization planning to curtail continuing environmental and economic damage now offers an opportunity to apply lessons that have been learned and to reinstate natural processes that were disrupted or interrupted by the MRGO during the half-century of its operation. One surprising outcome is that the restoration program may turn out to be more commercially successful than the original navigation project, which was conceived as an agent of economic transformation. The U.S. Army Core of Engineers still does not acknowledge, even in the face of compelling scientific evidence, that the MRGO project was a significant cause of early and catastrophic flooding of the Upper and Lower 9th Wards, St. Bernard Parish, and New Orleans East during Hurricane Katrina. A modeling effort that removed the MRGO from the landscape, and restored the cypress swamps and marshes killed by the MRGO, reduced flooding from Hurricane Katrina by 80%. We conclude that the MRGO spelled the difference between localized flooding, and the catastrophe that killed 1464 people and inflicted tens of billions of dollars of property damages. If the MRGO-caused economic damages associated with Hurricanes Betsy and Katrina are combined with those of construction, operation and maintenance, and wetlands destroyed, then the total economic cost of the MRGO is in the hundreds of billions of dollars.

During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.