Section 2.3: The Mississippi River Levee System
Since the earliest European colonists began occupying the Lower Mississippi Valley, some features of the behavior of the Mississippi River were problematic. On the one hand, the tendency of the Mississippi River to flood has posed problems for human settlements and farms located on its floodplain. On the other hand, various dynamics of the Mississippi River system, including the results of major flooding events, have caused the channel of the river to move around over time—a phenomenon often referred to as channel migration. For these two related reasons, human populations have had a long history of building earthworks to prevent problems of flooding, the silting up of the riverbed, and channel migration.
As discussed earlier in this book, the key characteristics of the Mississippi River in its delta is its tendency to accumulate sediment. This is what causes the Mississippi River to switch channels periodically and it is also responsible for the phenomenon of channel migration. Over time, sediment accumulates in the bed of the river as it forms bars, in this case specifically referred to as point bars. Point bars accumulate in the slowest-moving portions of the river channel, where sediment falls out of suspension in slack water on the inside bend of the river. On the outside of river bends, where water is flowing more rapidly, there is the erosion of the river bank. These two forces happen in tandem with one another, leading to lateral migration of the river channel.
Channel migration is obviously a threat to any human settlements on the banks of a river like the Mississippi. Similarly, the accumulation of sediment in the channel bed poses problems for river navigation, especially for the large sea-going barges that traverse the lower course of the Mississippi River. Thus, human populations living in the Lower Mississippi Valley have had to contend with this combination of problems and have often done so through large-scale engineering solutions.
The earliest strategy for dealing with the flooding of the Mississippi River was the location of settlements on natural levees of the Mississippi. New Orleans, for example, was purposefully located to take advantage of a natural levee on the East Bank of the Mississippi River (Campanella, 2008), which continues to be relevant to the city’s flooding issues today. In addition, early settlers protected agricultural lands through a patchwork of ad hoc levee construction, which combined private, municipal, and state oversight. It is even possible that New Orleans was protected by one such ad hoc levee as early as the 1719.
By the early 19th century, human-made levees had been constructed across a large section of the Lower Mississippi Valley. This allowed the reclamation of much of the agricultural land adjacent to the Mississippi River by preventing runoff from annual small-scale flooding events into adjacent low-lying wetlands. The construction of levees also stabilized the location of the channel of the Mississippi River. In this way, early levee construction opened the rich floodplains of the Lower Mississippi River for agricultural development.
The other key problem was the silting up of the Mississippi River channel. The solution to this problem was implemented by the engineer James B. Eads, who had constructed a major bridge across the Mississippi at St. Louis prior to the Civil War. Eads directed the construction of a system of jetties at the mouth of the South Pass of the Birdfoot Delta of the Mississippi River. These jetties sped up the flow of water through the South Pass by constricting the water flow through a narrow channel at the center of the river. This faster-flowing water scoured the silt at the bottom of the channel, deepening the mouth of the river for river traffic.
In 1882, the Mississippi River Commission implemented a “levee-only” flood control policy that sought to clear sediment and prevent further silting of Mississippi River channel by fully enclosing the Lower Mississippi Valley
river channel with a complete levee system. This approach would prevent the Mississippi River from losing water velocity as flood waters went over bank. Instead, as flood waters rose, they would be contained within the levee system, speeding them up and scouring out silt in the river channel. Over the following decades, and despite a series of catastrophic floods that destroyed large sections of the levee system, the Mississippi River Commission succeeded in completely enclosing the Lower Mississippi River channel with a continuous levee system.
On the one hand, these actions did greatly improve the navigability of the Mississippi River to ship traffic. By the turn of the 20th century, New Orleans had become one of the largest ports in the world and the Port of New Orleans remains one of the major drivers of economic activity in the city today. On the other hand, the construction of the continuous levee system had unintended consequences in terms of the effects of major floods. Since water was no longer allowed to escape onto the floodplains of the Mississippi River during floods, the levels of water in the channel rose dramatically. This led to a series of costly floods throughout the end of the 19th and early 20th century.
By far the most deadly and costly flood in the Lower Mississippi Valley occurred in 1927—an infamous disaster that continues to raise troubling questions about the intertwining of state and federal infrastructure, the environment, race, and class (Barry, 1997). The 1927 flood inundated 162,000 homes, creating more than a million refugees across the Deep South. As rising flood waters seemed to threaten the city of New Orleans during the summer of 1927, city officials dynamited levees downstream in Plaquemines Parish. In theory, this action was designed to relieve pressure and lower water levels in the Mississippi upstream at New Orleans. In practice, floodwaters were already in the process of receding by the time that the dynamiting took place, with pressure relieved by the failure of numerous upstream levees.
The dynamiting of levees in Plaquemines Parish during the 1927 flood resulted in catastrophic damages to farms and residences in the region, which were never significantly compensated (Barry, 1997). This damage disproportionately affected black farming and fishing communities, who lacked the political standing to necessary object or resist in a meaningful way. Thus, like many future disasters, the effects of the 1927 flood were structured by human modifications of the landscape and dynamics of political and racial inequality.
The public outcry after the 1927 flood led to the final and most complete federalization of the Lower Mississippi Valley flood control system. In order to avoid similar situations in the future, the U.S. Army Corps of Engineers constructed the West Atchafalaya, Morganza, and Bonnet Carre floodways. The former two spillways divert floodwaters from the channel of the Mississippi into the Atchafalaya River basin, which was formerly the main channel of the Mississippi. The Bonnet Carre spillway allows floodwaters to flow from the channel of the Mississippi into Lake Pontchartrain. In general, the idea behind the construction of these spillways was to relieve pressure on the Mississippi River levee system, which now completely encompassed the river from Rock Island, Illinois, to the Gulf of Mexico.
Today, the Bonnet Carre spillway is extremely important to the New Orleans metropolitan area and other settlements located downstream along the Mississippi River. During severe floods of the Mississippi, the Bonnet Carre spillway can be opened, allowing water to flow directly from the channel of the Mississippi into Lake Pontchartrain. Since the elevation of the water in the river is higher than the water in the lake, floodwaters essentially flow downhill into the lake and eventually out to sea. Thus, the Bonnet Carre spillway has helped to prevent flooding in the New Orleans metropolitan area since its construction in 1937.
Since 1928, the management of the levee system by the U.S. Army Corps of Engineers has been fairly successful in preventing floods and maintaining navigable waterways in the Lower Mississippi Valley, though the opening of the flood relief spillways often remains controversial. For example, in 2019, the Mississippi River one if its experienced its longest and most extreme periods of flooding, remaining at flood stage in Baton Rouge for a record 211 consecutive days. As a result, the Bonnet Carre spillway was opened twice in 2019 by the U.S. Army Corps of Engineers, with freshwater allowed to flow into Lake Pontchartrain and eventually the Gulf of Mexico for 118 days.
The environmental impact of the 2019 Bonnet Carre spillway opening was disastrous for both Lake Pontchartain and adjacent regions of the Gulf Coast. The large influx of freshwater from the Mississippi River, along with fertilizers and other pollutants, caused major die- offs among many saltwater fish and shellfish species, as well as dolphins and sea turtles. In addition, the decrease in salinity in combination with the increase in fertilizer nutrients led to a bloom of toxic algae in Lake Pontchartrain and the Mississippi Sound, which also had a large negative impact on local marine wildlife. While all of the effects of the 2019 Bonnet Carre spillway are still being assessed, it is already clear that they have caused great harm to both marine ecosystems and the fishing communities that depend on them.
Go to: Section 2.4: Impacts on the Ecology and Geology of the Gulf Coast