Stemming the Tide: The Mississippi River Delta and
the Davis Pond Freshwater Diversion Project
When I take my boat out fishing, I can't believe
my own eyes anymore. I have a GPS system and
depth reader on board, and I can't tell you the number
of times that the GPS tells me I should have been run aground.
But there I am, still floating four or five feet above the bottom.
That's when you know the problem is serious.
Louisiana Governor Mike Foster
Coastal Summit Conference
New Orleans, Louisiana
August 15, 2001
The March 26, 2002, dedication ceremonies for the Davis Pond Freshwater Diversion Project in Luling, Louisiana, marked the facility's opening with relatively little fanfare. Of course, there were ribbons cut, the ceremonial start-up by Louisiana Governor Mike Foster, and the regional news vans crowding the muddied, makeshift parking lot. But for a nation with plenty of other matters on its mind, the event went largely unnoticed. Nevertheless, the $119.6 million joint venture between the U.S. Army Corps of Engineers and the Louisiana Department of Natural Resources is, in fact, the largest freshwater diversion project ever built. As impressive as the Herculean ecological challenges that the Davis Pond project is set to address is the unique transitional role it will play between the patchwork of smaller coastal restoration projects that already dot Louisiana's coastline and the mammoth projects scheduled to get under way in the next few years. But perhaps most important, even if unnoticed by the outside world for the time being, is the way Davis Pond signals Louisiana's newly aggressive "do or die" attitude to what many have called the most severely compromised ecosystem on the face of the earth.
The Historical Mississippi River Delta
Since the dawn of geologic time, the globe has seen innumerable river systems come and go. Although their sizes, morphologies, and ultimate fates have been dictated by the large-scale dynamics of continental drift, shifting tectonics, climate change, and fluctuations in sea level, all river systems require are a relatively simple set of prerequisites: a partially elevated land mass, a depositional basin, and, of course, the rain and snow melt necessary to set the system in motion. And as long as there have been rivers, there have likewise been deltas, the nearly flat alluvial tracts of sediments that fan outward in triangular shape from a river's mouth. The Greek letter "Δ," in fact, inspired the coinage of the term "delta" by the Greek historian, Herodotus, in 450 BCE as he pursued his investigations of the land forms at the mouth of the Nile that so fascinated him.
Well prior to the classical era, however, humans had already shown considerable interest in deltaic systems because of their singular importance as fertile agricultural lands. The extended agricultural base they provided allowed human settlements to concentrate in ever-larger numbers, and across the centuries the reliable annual harvests spurred several civilizing impulses. Not only did burgeoning commerce systems and centralized markets flourish near these agricultural Edens, they also called for the creation of supporting infrastructure such as roads, irrigation, mills, storage facilities, and the like. In short, deltas served as a crucial foundation in the long human march from settlements to city-states by making possible the very idea of permanence.
The Mississippi River Delta, upon which New Orleans and much of the surrounding region sit, is no exception. History books do not paint ancient Americans' cultural connection to Mississippi delta with the same zeal as they do the influence of the Nile's delta system upon Egyptian culture. However, when De Soto's expedition first happened upon the Mississippi's debouchment into the Gulf of Mexico in 1543, Native Americans had been living and prospering there for some 12,000 years. In fact, as recently as the 13th century, the Mississippi River Delta was a thriving network of roads, commerce, towns, and towering earthen monuments. And once again, it was the delta's life-giving alluvial soil and its annual bounty of beans, squash, and corn that provided the sense of permanence necessary for this astonishingly complex civilization to develop and prosper.
Of course, the Mississippi River Delta these earliest of Americans called home, predated even their arrival and has a dynamic history all its own. Viewed through the long lens of geological time, the Mississippi's deltaic history resembles a lengthy battle between two well-matched rivals, each of which has seeming moments of victory only to see the tide turn-at times, literally-upon them once again. In geological terms, these rival forces are the epochs-old yin and yang of glacial periods, and, in the current glacial retreat we find ourselves living in, those of the delta's constructional and destructional phases that have come to be known as the "deltaic cycle." During the Late Wisconsian glaciation's peak roughly 22,000 years ago, sea level was 300-350 feet lower than present. Where the river met the sea, its overloaded flow braided (i.e., branched and reunited), cutting deep valleys into the Pleistocene subsurface, the aggraded remnants of which were deposited at the river's mouth in the form of gravel deposits and coarse sand. When the Late Wisconsian glaciers began melting some 4,000 years later, an era of sea-level rise known as the Holocene transgression was ushered in, and with it came more dramatic changes in the river's hydrologic character.
As sea level rose, the Mississippi changed from a braided system carrying coarse-grained deposits to a sinuous, meandering system with distributaries that would form only to be abandoned, a system in which the erosion of fine-grained soils in the floodplains were delivered to the river's mouth. These topstratum sediments were deposited atop the coarse-grained substratum beneath, infilling the incised valleys. Once the valleys became packed with sediment, the distributary would of necessity shift course, moving beyond its previous constraints.
This "distributary switching," with a periodicity of 750-1500 years, defines the deltaic cycle. When distributaries are present to deliver sediment, deltas are able to accrete building materials and prograde seaward, as do the marsh plants that colonize upon them; when a distributary is abandoned, however, the delta that has been built up begins to subside from compaction and erode in the face of tidal energies. The historical pattern of switching, accretion, and subsidence has left a complex developmental record behind. Nevertheless, geologists have used radio carbon dating surveys to identify seven well-defined, yet often overlapping, "lobes" within the larger Mississippi River Delta system. New Orleans, for example, sits upon the overlapping portion of the Cocodrie and St. Bernard lobes. The former was the product of a 1,000-1,500 year long building phase that took place between 2,600 and 1,000 BCE, while the latter was built across a 1,500 year period only 2,000 years ago.
In the far shorter view of post-European settlement of America, the Mississippi River Delta has largely had the upper hand. Although the cycle of accretion and subsidence continued to play out, the delta was in a "net accretion," or building, phase, and by the time De Soto's company encountered the Mississippi, the delta had expanded to encompass over 5,000 square miles. Those sediments that didn't carry to the delta's outermost edges to expand its reach were layered closer inland atop those of previous years, where they supported not only marshes but cypress-tupelo swamps as well.
New Orleans' Regional History
The problems Davis Pond is designed to address have their roots deeply imbedded in New Orleans' regional history. From its settlement at the turn of the 18th century, embankments were constructed in order to protect the low-lying town from Mississippi floodwaters. Although the land closest to the river is elevated relative to the deltaic plain as a whole, these embankments would still be overtopped often enough to create periodic disasters for the area. By the turn of the following century, a network of some 100 miles of levees had been established, but between the construction methods of the day and the river's sheer force, breaches in the network were common. Although they created havoc for the city, these same breaches--historically known as "crevasses"--eventually became associated with one benefit in particular, namely increased catches by local fishermen.
The crevasse flows that proved a boon to area fishermen were soon discovered to have positive impacts beyond the fishing docks. In the early 20th century, the agency that would become the Louisiana Department of Wildlife and Fisheries understood that crevasses brought about a "restoration of wetlands . . . akin to the cultivation and fertilization of farmlands." But because they carried fresh water, nutrients, and sediments, the crevasse flows were not simply "akin" to fertilization but were, instead, the genuine article. Yet, the waters that carried them were one and the same with those that the levees were designed to hold back.
Despite these precautions, there was extraordinary, region-wide flooding in 1927. Subsequently, the U.S. Congress funded a sustained effort to further improve the Mississippi's levee system. But even as they raised levees, they began to move away from the "levees only" approach, creating bank overflow structures that, in a sense, are the ancestors of the Davis Pond project. As flood control methods improved, so did the sense of security afforded to New Orleans' residents, merchants, and manufacturers. And as families, resources, and infrastructure concentrated in the region, the need to "insure" against their catastrophic loss increased accordingly. As new technologies developed, levees were structurally reinforced, increased in height, and augmented by the addition of pumping stations. In short, a human-driven cycle had begun to compete with the ages-old natural cycle.
The network of interlaced navigational canals established throughout southeastern Louisiana across the last century has taken its toll on wetlands as well. Whereas the levees lessen the flow of fresh water into wetlands, channels dredged for commercial shipping and oil extraction have created new pathways for the flow of salt water into the coastal interior. In the latter half of the 20th century, a pattern emerged. Areas once dominated by freshwater marshes experienced conversion to more salt-tolerant species. Brackish marshes converted to saltwater. The heightened salinities killed off the existing vegetation, causing the root systems that bind the submerged soil to decay. And because the yearly sediment deposits were no longer there to counter the ongoing wind and wave erosion at the delta's edge, soil substrates anchoring exposed marshes were simply washed away, converting once functional wetlands into open water.
America's Working Wetlands
The reason there is so much concern over the state of Louisiana's coastal wetlands is simple: they are extremely valuable resources. And while they are indeed the national treasures that environmentalists and cultural preservationists so ardently defend, they are exceedingly valuable assets in the purest economic terms as well. Given that Louisiana residents are just now coming to understand that wetland loss, in fact, amounts to the dismantling of their state's primary economic engine, it comes as no surprise that the country at large is unaware of coastal Louisiana's role as one of the nation's leading economic powerhouses.
Fisheries
The harvesting of fish and shellfish off Louisiana's coast is a prime example of the region's importance to the American economy. While this includes thousands of "Mom and Pop" enterprises that sell their catches to regional restaurants, it also includes big business. Really big business. To the tune of billions a year, Louisiana's commercial fisheries ship products not only across the length and breadth of the United States, but to every portion of the globe.
Year in and year out, Louisiana's coastal fisheries average roughly 30% of the total annual landings in the contiguous United States. A top producer of shrimp, Louisiana also ranks first in the nation when it comes to the harvest of oysters, crabs, and menhaden. While the latter species is easy to overlook because it is not usually consumed by humans, the nearly billion pounds of menhaden landed each year is processed into some 400 million pounds of highly nutritional fish meal, oil, and other solubles. These, in turn, provide absolutely critical, low-cost ingredients to a global array of industries that produce cattle, poultry, and hog feed.
All told, approximately $300 million a year is paid dockside to Louisiana's coastal fisheries. But the value generated doesn't end there. Rather, the subsequent chain of wholesale, manufacturing, distribution, and retail activity leads to a "total economic effect" (the term economists use when calculating the ripple effect that primary economic activities such as fish landings create across the nation's economy) of somewhere between $2.5 and $3 billion per year.
Although much of the fishing and netting is done in deeper waters, catches would be far smaller were it not for Louisiana's coastal wetlands. Of the species that make up Louisiana's commercial fishing industry, 90% of them spend some portion of their lives in estuaries and marshes, relying upon the dense grasses for both nourishment and protection from predators. In fact, scientists have found that the amount of ocean-to-wetland interface in estuarine systems is one of the best overall predictors for the population numbers of the species that rely upon them. Given the current projections that 20% of Louisiana's remaining wetlands will be lost by the year 2050, the loss in total economic effect will likely escalate to roughly $500 million per year by mid-century.
In more humanitarian terms, this equates to 220 million pounds of fish protein--be it directly consumed or fed to livestock--that will be lost to the forthcoming generation. This loss, when coupled with the increased demand for protein from an ever-increasing global population, can only mean higher costs for everyone. And in developing nations, the price tag may well be so high as to include starvation.
Ports, Navigation and International Shipping
The port and waterway system in south Louisiana is arguably one of the most extensive on the face of the earth. It is, without a doubt, the world's busiest. Made up of over 3,000 miles of interconnecting rivers, channels, canals, and anchorages, the system handles roughly 20% of the United States' international waterborne commerce, with the port complex between New Orleans and Baton Rouge alone boasting the largest movement of tonnage in the world. Moreover, this same complex, in the parlance of the shipping industry, is "intermodally connected." In other words, it provides ready access to 50 ocean carriers, 75 truck lines, 16 barge lines, and is the only U.S. port to be served by six "class one" rail carriers.
States within the nation's middle corridor are especially reliant on south Louisiana's ports. Economists have estimated that over 40% of the markets in the American Midwest rely on their ability to move goods in and out of facilities at the mouth of the Mississippi. This reliance is clearly demonstrable as far north as Minneapolis, where, in a single recent year, ports there exported 70 million tons (mostly food products) and imported 60 million tons (mostly petroleum products) through Louisiana ports.
Yet the very network of canals and waterways that makes this thriving economic activity possible has taken its toll on the landscape that supports it. The very task of moving cargo from the open waters of Gulf of Mexico into Louisiana ports calls for numerous, north-south trending waterways. This geographical necessity, however, creates a number of unwanted consequences. One, touched on above, is the way navigation channels allow damaging salt water from the gulf ready access to the wetlands they cut through.
But the same pathways that allow regular tidal pulses to deliver salt water inland turn into even greater threats when tropical storms and hurricanes pass through the region, becoming what some have called "storm surge superhighways." In these instances, not only does the cycle of salinity spikes, vegetation dieback, and substrate erosion play out in areas even further inland, but impounded areas without sufficient drainage can be left with elevated salinity levels for years to come. And since humans live and work in these inland areas, canal-born storm surges can threaten industry, infrastructure, homes, and lives.
Oil and Natural Gas
Perhaps nothing makes a better case for the importance of Louisiana's coastal wetlands to the U.S. economy--and, hence, the redoubled efforts to protect and restore them--than the nation's oil and gas industry. Including the federally owned waters off the Louisiana coast, there is in excess of 20,000 miles of large to medium diameter pipelines crisscrossing the region. Pipelines connect offshore production platforms to land- and water-based natural gas and oil storage facilities, then connect these facilities to 30% of the nation's refineries and a broad range of petrochemical manufacturers. Other pipelines transfer crude, refined, and processed petrochemicals to Louisiana ports, tanker-truck fleets, and rail carriers for transport. Still other pipelines link networks of natural gas wells to the nation's most critical distribution hub.
Also tied into this network is America's Strategic Petroleum Reserve (SPR). Used to store an emergency supply of 560 million barrels of crude oil, two of the nation's four SPR complexes are found in coastal Louisiana near the towns of West Hackberry and Bayou Choctaw. Stored in hundreds of below-ground salt domes that are each typically large enough to contain the Sears Tower, the full capacity of the SPR system currently stands at 700 million barrels, representing a maximum inventory protection of 118 days' worth of average national consumption. In total, the SPR represents a $20 billion national investment, a good portion of which resides in coastal Louisiana.
The price tag on this infrastructure, both commercial and federally owned, is estimated to be somewhere near the $100 billion range. But that figure dwarfed by the cumulative value of the commodities it has produced and moved across the years. The value of Louisiana's annual crude oil production, for example, has been hovering somewhere near the $10 billion range for the past several years. The state's 19 refineries produce 23% of the nation's domestic oil and gas, accounting for 15% of the nation's overall refining capacity (i.e., domestic and imported crude processing combined). Together, they produce $17 billion worth of petroleum products per year. As for natural gas, Louisiana is responsible for 27% of U.S. production, some $10 billion worth per year. Much of it passes through the Henry Hub facility just south of Abbeville, a distribution center for over a third of the natural gas delivered within the United States. In fact, the Hub is deemed so critical to the natural gas industry that its daily volume alone is used to determine market prices nationwide.
New technologies have made it much easier to find deep-water oil prospects, and throughout the 1990's even seasoned industry experts were stunned by the amount of untapped resources discovered just off the continental shelf. Yet the ability to identify and successfully remove this vast supply of energy ultimately means little without a reliable means of getting it to shore. And indeed, there is mounting concern about the health of the pipelines now being used to bring the oil to Louisiana's coast for processing and distribution. This concern is due only in small measure to the age of the pipelines. Instead, it stems from the rapidly deteriorating barrier islands and other hazardous terrain they must traverse. This degradation of the landscape not only increases exposure of pipelines to the periodic ravages of storms and hurricanes, but to the constant erosive force of waves that now travel unimpeded across vast stretches of open water that were once sheltered. This endangered infrastructure comes at a bad time for a nation increasingly reliant on the oil deposits just off Louisiana's coast, for while the state's share of total U.S. production was 15% in 1990, it has increased sharply across the last decade, currently standing at 23% of the nation's production.
One Possible Future
Belying it's "lazy days on the bayou" image, coastal Louisiana has to be considered among a select handful of the nation's key production and trade centers. Now imagine all of this--the busiest port in the world, over a third of the nation's natural gas supply, much of the nation's strategic oil reserve, hundreds of thousands of lives that live and work in Louisiana's 18 coastal parishes--under the dire threat of attack.
Now stop imagining. Because all of this is indeed imperiled.
The nation has lost more than 600,000 acres of vegetated coastal wetlands in coastal Louisiana over the past century. The present rate of annual loss, despite over a decade's worth of efforts to mitigate the problem, is a staggering 25-35 square miles per year. That's 440 acres a week. An area the size of 40 football fields per day. If this loss is not addressed head on, the future of Louisiana's coastal communities is bleak. Even without a catastrophic event, the steady chipping away of the coastal zone spells nothing but trouble. But one only needs to look at coastal Louisiana's experience with floods and hurricanes across the past century to understand that, unfortunately, holding out hope for a "catastrophe free" future amounts to little more than whistling past the graveyard.
As far as their roles in times of floods are concerned, one only needs to think of wetlands as natural reservoirs to understand their preventative role. Not only do they absorb the runoff caused by direct, inland rainfall, but they also lessen the degree that the wind-driven tidal surges accompanying severe storms can make their way into the state's interior. In fact, research has shown that every 2.7 miles of marsh that a tidal surge has to push its way across lowers the surge height by one foot.
Of course, there is that class of especially severe storms known as hurricanes. Statistics show that a major hurricane (i.e., a Category 3, 4, or 5, with winds of at least 111 mph) strikes coastal Louisiana roughly once every eight years. Because hurricanes are fueled by the uptake of warm, moisture-laden air from directly above the sea, wetlands, especially the forested variety, help put the brakes on winds, explaining why true hurricane force winds of 75 mph or better are rarely seen 50 miles inland. But as 1998's Category 2 storm Hurricane Georges proved, even hurricanes packing less of a punch in terms of wind can still pose serious threats by virtue of the storm surges they produce. Pushing a 17-foot high wall of water in front of it, Georges came perilously close to pumping this surge directly into Lake Ponhcartrain. Had it done so, the results for New Orleans, the majority of which lies below sea level, would have been devastating.
The mind boggles, then, to consider the results should a Category 5 storm such as 1970's Hurricane Camille and the 210-mph winds that savaged Mississippi's coast were to hit the New Orleans area directly. The narrow ribbons that are Louisiana Highways 1 and 23, both of which service the port and oil production complexes south of New Orleans, would likely have lengthy sections entirely washed away, leaving those industries compromised for quite some time. The exposed oil infrastructure and the surrounding environment would certainly suffer as well. A breach in but one single section of 24-inch pipeline running at capacity would pump 2.5 million gallons per hour into the surrounding area. Given the gulf wetlands' role as fish nursery grounds, if such breaches were multiplied just a handful of times over, the long-term impact on the fishing industry would be astronomical.
As far as New Orleans itself, the news would be far worse. In many of the hurricane models produced by scientists at Louisiana State University, the limited exit routes leading north from the city would be cut off, stranding hundreds of thousands to face up to 20 feet of water. While the city of New Orleans has 10,000 body bags stored in its emergency management facilities, these same scientists have generated models wherein as many as three, five, and perhaps even ten times that number of bags might ultimately be required to remove the dead that a severe storm would leave behind.
While the times we live in can't help but dominate our conception of the world, it is important to emphasize that despite its location, coastal Louisiana has not always been in this plight. The loss of 40 football fields' worth of wetlands day in, day out, year after year, simply was not the case at the turn of the 20th century. And each day this loss goes unabated is tantamount to asking for elevated storm surges, more ferocious winds, and heightened economic loss. Not to mention a greater human toll.
The Davis Pond Freshwater Diversion Project
Given its importance to the future of the Barataria Basin and coastal Louisiana's restoration efforts, the Davis Pond structure is strikingly pedestrian at first glance. In fact, the basic design concept was developed in the late 1960's under authorization by the Flood Control Act of 1965. But it wasn't until Louisiana's coastal restoration efforts came of age in the late 1980's that the project went from the drawing board and, in 1997, into construction under the auspices of the U.S. Army Corps of Engineers. A similar project that was also built by the Corps, the Caernarvon Freshwater Diversion, has been on line for over a decade now and provided the construction team with an understanding of successful and not-so-successful strategies alike. Capable of diverting up to 8,000 cubic feet per second (cfs), Caernarvon freshens the Breton estuary south of New Orleans, nourishing emergent vegetation in the estuarine marshes and bays.
Built 22 miles upstream of New Orleans into the west bank of the Mississippi, Davis Pond's structural specifications include a concrete inflow channel roughly 500 feet in length that will direct the flow of water into four 14 x 14 feet, iron-gated box culverts built directly into the river levee. These gates will be capable of passing up to 10,600 cfs, a capacity roughly one third greater than that of Caernarvon. A 120-foot wide outflow channel running some 11,000 feet will connect the structure to a 9,300-acre ponding area that, in turn, will allow the water to pass over a lengthy rock weir and into its primary targets in the Barataria basin: first Lake Catouache, with its adjacent swamps and marshes, then into the much larger Lake Salvador system.
Despite the vintage of its original conception and the precedent of Caernarvon, the Davis Pond project is truly a new chapter in Louisiana's coastal restoration efforts for a variety of reasons. Aside from its increased discharge capacity, one of the things that sets the Davis Pond project apart from previous diversions is that the chief targets of benefit are the forested swamps that the diverted water flows directly into. Another is the large holding pond, placed into the chain of flow so that something of a stabilizing buffer is built into the outfall management capability. Still another unique aspect of the diversion is the rock weir running between the holding pond and Lake Catouache. The nearly mile long arc of weir was created so that rather than simply finding the lowest point between the two bodies of water and eventually "channelizing" the outflow, the water exiting the pond will instead be encouraged to "sheet" across the barrier, promoting a wider, more even distribution at the outset of the water's movement southward through the estuary.
The ponding area, in particular, represents an entirely new wrinkle in the approach to diversions. A common misconception, given the important role that sedimentation plays in the delta building process, is that Davis Pond will move large amounts of sediment into the Barataria basin. While such would be the case given more massive, uncontrolled flooding, Davis Pond is exactly as advertised: a controlled freshwater diversion. While in one sense the idea behind the project is to simulate historic flooding, the sensitivity to the adjacent populace has brought the operational terms "controlled diversion" to the forefront of the project team's concerns.
Likewise, while the project will no doubt deliver some amount of sediment and nutrients to the estuary, its true purpose, as stated, is to deliver fresh water. In fact, one of the holding pond's key jobs is to allow for the settling of sediments prior to the water's passage over the threshold of the weir. Sands and heavier sediments will settle out to the pond's bottom, creating the need for a dredging program that will help maintain the pond's holding capacity. Lighter solids such as the suspended clays and silt that make up 80-90% of the river's sediment inflow will probably make their way across the weir, but not before a good measure of these are filtered out by the thick-mat floating marsh (Panicum hemitomon) that carpets portions of the ponding area so heavily that average-sized adults can walk across it as they might a deep waterbed.
One might well ask, "Why not deliver these sediments into the estuary?" The answer, from the ecologists' point of view, is that nutrients, especially nitrogen, are adsorbed onto the outer surfaces of these particles, ready to fuel a boom in plant growth. The next question might well be, "What's wrong with nutrients entering the estuary?" This time the answer comes in one word: eutrophication. When the nutrient availability within an aquatic system becomes hyper-enriched, limits to plant growth all but disappear. While this, too, may sound like a desirable effect at first, it can present a problem if the system's flow rate is so slow that water in the system does not "turn over" rapidly enough. It is important to recognize that not all plant life is of the leafy, woody variety that can be seen with the naked eye. Rather, because their life cycles are defined by days rather than seasons, algae and associated phytoplankton are among the first to utilize these nutrients when they appear in the system, and the result is a sudden, explosive "algal bloom." Upon the inevitable demise of the plant life within the bloom, bacterial decomposition of the algae depletes the system of oxygen at a similarly rapid rate. This low oxygen or "hypoxic" state has drastic effects upon the food chain as animals--from microscopic zooplankton to fish--will die off, furthering the hypoxic cycle as they, too, begin to decompose. Given that the Mississippi runs through the nation's "breadbasket" and is exposed to agricultural runoff with high nutrient loads, the pond serves as an important control against estuarine eutrophication.
The ponding area will also serve to control another important aspect of operation, namely the stabilization of flow rates. Under certain scenarios, such as high winds, the stage on the inside of Caernarvon can rapidly change by one to three feet. With the ponding area included in the Davis Pond design, however, the water will act as a buffering reservoir of sorts, allowing managers more precise control over the day-to-day outfall rate while lessening the impact of anomalous, weather-induced fluctuations. And ultimately, it is the fine degree of control that managers will have on the system--and, hence, upon an unprecedented basin-wide scale that may prove to be ten times the size of Caernarvon's outfall--that may well prove to be Davis Pond's most defining characteristic.
Monitoring and Modeling
Although Davis Pond is capable of diverting over 10,000 cfs, in practice, its flow rate will be far lower and governed by a strict set of operational guidelines. Many of the factors that help to frame these guidelines come by way of simply measuring the ecosystem trends the project is intended to shape: the number of fish and shellfish the estuary will produce; water levels; water quality; the degree of revegetation; and perhaps most important, salinity levels. To this end, an extensive monitoring program has been established by the Louisiana Department of Natural Resources, several components of which have already been in place for years in order to provide a baseline by which future benefits can be accurately measured.
In keeping with the correlation between crevasse and fish catches first observed nearly a century ago, the number of fish landings will be one important aspect of the Davis Pond monitoring program. Managers plan to collect their data by sampling commercial creel holdings, making monthly aerial assessments of the number of commercial fishing vessels working the area, and examining the yearly landings data gathered by wholesale dealers. In addition, 15 in-shore and four offshore trawl stations are set to monitor shrimp weekly or bi-weekly at various times in the year. And in the weeks immediately before brown shrimp come into season, four additional stations will be employed to carefully scrutinize the commercially vital populations.
The success of both the fish and shellfish restoration efforts, of course, rests largely in the quantity and quality of the water introduced into the Barataria Bay estuary. Prior to construction, water level and velocity were the subject of continuous monitoring at nine sites. Once the project comes fully on-line, the number of water level sites expanded to 13 and those measuring velocity were brought up to 19. Likewise, the overall water quality of the Davis Pond outfall is extremely important to the project's long-term success. While there is a public perception that the Mississippi is polluted to the point of toxicity, regulatory efforts across the last 30 years have slowly but surely cleaned up the river to the point that it is far less polluted now than it has been for decades. Although nutrient loads can still be a problem, the presence of DDT, PCBs, petroleum by-products, and non-ferrous metals such as mercury and lead have declined sharply. Preferring to err on the side of caution, however, Davis Pond project area monitoring includes monthly sampling at 20 stations to analyze 30 different water quality parameters, including the presence of fecal coliform bacteria and a variety of pesticides, herbicides, nutrients, and metals. As an adjunct to these measurements, tissue and sediments are being sampled for the presence of contaminants as well.
Restoring the sustainability of natural vegetation-in large measure, Davis Pond's whole reason for being-is a critical component that is being carefully monitored as well. Not only will the extent of vegetative cover graphically reflect the overall degree of marsh recovery, it will also indicate the amount of nursery habitat the area can provide juvenile fishes. Several dozen north-south transects across the basin will be surveyed after the first, third, fifth, and seventh years of operation, followed by surveys every five years thereafter. Low altitude photos of the area, a "benchmark" set of which has already been taken, will also be utilized.
As we have seen, rather than depending on sediments or nutrients to preserve and restore the swamps and marshes of the Barataria Basin, that job will be left almost exclusively to the fresh water directed into the system. By simply freshening the estuary--that is, by reducing salinities and reestablishing an increased hydrologic flow-through--vegetated areas that have come to stress-induced standstills or saline-driven diebacks will be revegetated. This renewed growth of existing plants will enable them to better keep up with sea-level rise, and the emergent vegetation will help to create the extensive root networks that bind the soil, making the substrate more resistant to erosion. Most important, perhaps, is that as these heartier plants reclaim their historic range within the basin, they will produce nutrient-rich peat through their seasonal decay and decomposition cycles. In essence, this new vegetation will begin producing the very substrate subsequent generations of plants will rely upon, producing both an accretion and, as the marshes colonize seaward, progradation effect.
The extent and rate the estuary recovers its biomass, therefore, will be one of the most important means managers' will have of evaluating the ultimate success of the Davis Pond effort. An important adjunct to this monitoring program will be data provided by the USGS National Wetlands Research Center's remote sensing capabilities, one of several partners lending their expertise to the project. Using orbiting satellite platforms and LIDAR analysis (Light Detection and Ranging), leaf, canopy, and grass blade spectral reflectance data will be generated. Because emergent and budding vegetation present different spectral characteristics, this data will go beyond the colonization range data provided by aerial photography to examine the extent of new growth within established vegetative tracts. Scientists at Louisiana State University, another partner to the project, are also looking at the question of vegetation, especially the amount and range of river-borne nitrogen appearing in the Barataria Basin. Because plants along the Mississippi's banks carry a characteristic N14/N15 "fingerprint" distinct from the nitrogen profile characteristic of the estuary, samples of plants across the project's outfall area are being analyzed. This will help managers understand how much nitrogen is being drawn from the river, the nitrogen outfall gradient across the estuary and its correlation to biomass production, as well as providing a watchful eye for the tell-tale signs of over-nitrification.
Of all the governing guidelines, however, salinity has to be considered the principal measure from which Davis Pond's flow will be determined. Like other monitoring aspects, salinity levels have been the subject of on-going scrutiny for a number of years, with 17 continuous recording stations across the basin providing pre-flow data. Once outfall from Davis Pond commenced, a full 31 additional stations came on-line, with four of these being specially designed data collection platforms outfitted with satellite uplinks so that salinities can be remotely monitored in real time. From these data, "isohaline lines" are being generated weekly (i.e., range-grouped salinity lines similar to those seen on contoured elevation or weather maps).
It is the pivotal issue of salinity upon which much of the Davis Pond project's computer modeling efforts have been focused. In essence, there are two essential points that the management team will be focusing on: the 5 parts per thousand (ppt) salinity line and the 15-ppt salinity line. Where these lines are ultimately determined to be optimal within the basin is still to be precisely determined, but managers have a good idea of where they should run based upon the salinity tolerance of the flora and fauna that range from the top of the basin all the way to the open sea. Even after they are fine tuned, however, they will be closely tied to the discharge rate moving through the structure's gates
Managing for these two distinct targets across the entire basin--the 5-ppt line being closer to the weir's overflow than the more seaward 15-ppt line--requires that the modelers and managers get into the "forecasting business," looking ahead several weeks so that they can anticipate winds, rains, river levels, and temperatures, as all of these will have in impact upon the accuracy with which they hit their salinity marks. As with the monitoring programs, the salinity model has been in the subject of study for several years, with weather conditions, flow simulations, and basin-wide salinity observations, along with the necessary tweaks to the model, occurring twice monthly the year prior to start-up. All of these observations, of course, were made pre-flow, and managers have only recently gained the opportunity to check the accuracy of their model and begin to make adjustments under actual conditions.
Along with this question of the accuracy, one aspect of the system at large the modelers and managers alike are most interested in is its "elasticity," that is, the rate at which the system responds when they make an adjustment. Of special interest is the rate at which the system corrects itself should there be an over-correction in the wake of a faulty forecast. The 5-ppt line is especially a matter of concern. Because it is closer to the river's outflow, it will be much more sensitive to changes in the discharge rate coming through the gates and overtopping the weir. The model suggests that the system is elastic, even at the 5-ppt line, and that it will bounce back from even fairly severe instances of over-freshening in less than two weeks. While they are just beginning to gather the data that will determine if this is indeed the case or not, if the system does prove to have the elasticity the model suggests, it will make the entire system much easier to manage.
2050 and Beyond
All told, ecologists are predicting that Davis Pond will directly preserve 33,000 acres of marsh that would otherwise be lost over the next 50 years, while benefiting over 750,000 additional acres by restoring the area to its more natural hydrology. As if tackling the issue of basin-wide restoration wasn't enough, Davis Pond is eventually expected to pay for itself. Economists estimate that the project will annually generate $15 million in fish and wildlife benefits and $300 million in increased recreation dollars. Even if these estimates are off by 50%, this means that Davis Pond will still pay for itself in roughly 15 years. And this, of course, does not include the untold benefits to the nation's economy that will be gained by keeping intact the Louisiana coastal zone's first line of defense against hurricanes.
When all is said and done, the $119.6 million dollar price tag looks more and more like bargain every day. More than an insurance policy, perhaps it is better to think of the Davis Pond effort as part of a larger assurance policy. After all, scientists agree that the question of a major hurricane striking the region is not so much a matter of "if" but "when." Assured that a major storm will hit Louisiana's coast at some point, then, we can also be assured that measures such as Davis Pond and the even larger scale diversions planned for the near future-no matter a given storm's intensity and the damage it causes-will help to mitigate the loss the nation would have seen had such projects not been undertaken.
After all these years of doing our best to tame the Mississippi, it is telling to see how far we've come towards understanding how we can improve our lot by simply letting the river do its job. And while we've come too far to return to the days of completely unfettered flooding and distributary activity that the aboriginal Americans knew, it now seems just as unlikely that we'll ever return to the days in which the river was viewed in largely adversarial terms. Indeed, in the long run, it may yet prove to be the staunchest ally we have.