Louisiana’s landscape, shaped by the powerful Mississippi River, occupies a distinctive position on the Gulf Coast. Its low elevation characterizes a significant portion of the state, making it susceptible to the dynamics of water. In this context, “below sea level” refers to land areas with an elevation lower than the average height of the sea surface, known as mean sea level. This unique geography is not static; it is a dynamic environment continually influenced by complex geological processes.
The Extent of Submerged Land
Establishing a precise, static figure for Louisiana’s land area below mean sea level is challenging due to the state’s dynamic coastal environment and ongoing changes, though extensive low-lying areas exist, particularly in the southern regions. Approximately 4.7 million acres of land are less than 10 feet above the local high tide line, indicating vulnerability. While some sources claim nearly half the state is below sea level, this figure is debated. Many developed areas, especially in the Greater New Orleans metropolis, have subsided below sea level.
Natural and Human Contributions to Subsidence
The sinking of land, known as subsidence, in Louisiana results from both natural geological forces and human activities. Naturally, the immense weight of sediments deposited by the Mississippi River over millions of years causes the Earth’s crust to flex downward. This process is compounded by the compaction of thick, unconsolidated deltaic sediments as water is squeezed out. The presence of numerous vertical faults in the Gulf of Mexico basin, resulting from salt movement and growth faulting, also contributes to subsidence.
Human interventions have significantly accelerated these natural processes. Extensive levee systems along the Mississippi River, intended to control flooding and aid navigation, have prevented the natural replenishment of sediment to wetlands. This lack of new sediment deposition means the land cannot naturally build up to offset ongoing compaction and subsidence. The extraction of subsurface resources, such as oil, natural gas, and groundwater, contributes to subsidence by reducing pore pressure and causing the compaction of reservoir rocks. Draining wetlands for development also causes underlying clay deposits to dry and shrink, leading to further land subsidence.
Louisiana’s Low-Lying Regions
Specific areas within Louisiana are characterized by their low elevation. The Greater New Orleans area, for instance, includes large urbanized portions of Orleans, Jefferson, St. Bernard, and Plaquemines parishes that are below sea level, with some neighborhoods like Eastover in eastern New Orleans lying 10 to 14 feet below. These areas are typically protected by extensive levee systems.
Beyond major urban centers, much of Louisiana’s coastal zone is inherently low-lying. The southern third of the state is only a few meters above sea level, with extensive salt marshes averaging less than one meter in elevation. Regions along Bayou Lafourche, from Raceland to Galliano, and areas near Houma and Montegut, have also experienced significant subsidence, placing them below the adjacent tidewater marshes.
Implications of Living Below Sea Level
The low elevation of much of Louisiana results in several inherent conditions. One primary consequence is an increased vulnerability to various forms of flooding. Coastal areas are particularly susceptible to storm surges, where strong winds push ocean water inland, and to heavy rainfall that overwhelms drainage systems. River overflow also poses a threat, as rivers can swell beyond their banks, especially in areas where the land is lower than the river itself.
Another direct outcome is the ongoing process of land loss and coastal erosion. Without sufficient sediment input, the existing land, particularly wetlands, naturally compacts and subsides, leading to its conversion into open water. This erosion is exacerbated by the constant movement of water and the impacts of storms. Additionally, saltwater intrusion presents a significant challenge. As land sinks and freshwater flow patterns change, saltwater from the Gulf of Mexico can advance inland into freshwater ecosystems, aquifers, and agricultural lands, affecting drinking water supplies and crop viability.