New Orleans is distinct among American cities because large portions of its inhabited land lie significantly below the average level of the Gulf of Mexico and Lake Pontchartrain. This low elevation creates a bowl-like shape where water naturally collects rather than draining away. The city’s low elevation is a consequence of its geological origins within a river delta and the lasting effects of human engineering efforts over the last two centuries.
How the Mississippi River Built the Land
New Orleans is situated on the Mississippi River Delta, a geologically young landmass created by thousands of years of sediment deposition. As the river slows near the Gulf of Mexico, it drops its load of sand, silt, and clay, collectively known as alluvium, forming a fan-shaped expanse. This sedimentation built up the land above sea level, primarily along the river’s immediate banks, forming natural levees that were the first areas of settlement.
The land away from the river, where much of the modern city is built, consists of soft, water-logged deltaic soils, including marshland and cypress swamps. This foundation is inherently unstable and easily compressed because it is composed of unconsolidated layers of fine sediment and organic matter. For millennia, annual floods deposited fresh layers of sediment across the delta plain, replenishing the land and sustaining its elevation against natural sinking.
The Mechanisms of Land Subsidence
The primary reason New Orleans is now below sea level is subsidence, the gradual sinking of the land surface. While natural compaction of the soft deltaic soil has always occurred, human activities have dramatically accelerated this process. Much of the metropolitan area is sinking at an average rate of 5 to 10 millimeters per year.
A major contributing factor is the extensive system of upstream flood-control levees built along the Mississippi River. These structures prevent the river from naturally overflowing its banks and depositing new alluvium onto the delta plain, a process known as sediment starvation. By cutting off the river’s natural land-building process, the levees removed the mechanism that once counteracted the delta’s inherent subsidence. This means that as the underlying sediment compacts and settles, no new material is added to raise the surface elevation.
Urbanization also contributed by draining surrounding wetlands to create developable land. Pumping water out of the soil lowers the water table, causing the soft, saturated organic-rich soils to dry out. When organic matter is exposed to air, it undergoes oxidation, a chemical process that causes it to decompose and shrink rapidly. This dewatering and oxidation leads to rapid compaction and a loss of ground elevation, with some old swamp areas settling as much as three meters since the late 19th century.
Maintaining a City Below Water Level
Keeping a city dry that is lower than the surrounding water requires continuous engineering efforts. The first line of defense is the Hurricane and Storm Damage Risk Reduction System, a perimeter of federal and local levees, floodwalls, and floodgates. This 133-mile system is designed to hold back the waters of the Mississippi River, Lake Pontchartrain, and the Gulf of Mexico, protecting the urban area from storm surge.
Because the city is below the water level, rainwater cannot drain away by gravity alone, necessitating a complex network of internal drainage infrastructure. The city’s drainage system includes approximately 200 miles of open and underground canals, which collect runoff from over 68,000 catch basins. This water is then directed to 24 drainage pumping stations, which house 120 pumps that mechanically lift the collected rainwater out of the city and over the protective levees, discharging it into Lake Pontchartrain or nearby waterways.
The scale of this pumping operation is enormous, capable of moving water at a rate of more than 45,000 cubic feet per second. This mechanical removal of water is required not only during rainstorms but also daily to manage groundwater seepage that constantly enters the drainage canals.