Lake Pontchartrain is neither purely saltwater nor purely freshwater, but is correctly classified as a brackish estuary. The immense body of water, which covers over 630 square miles, represents a dynamic mixing zone where freshwater runoff from a large watershed meets the salty influence of the Gulf of Mexico. This constant blend creates a unique and fluctuating environment, which is why the lake’s official classification is an estuary, despite its name suggesting it is a freshwater body. The salinity levels shift dramatically based on season and weather, making it an environment of perpetual change.
Defining Brackish Water
Brackish water describes any aquatic environment with a salinity content greater than freshwater but less than seawater. Salinity is measured in parts per thousand (PPT), which refers to the concentration of dissolved salts. Freshwater, such as that found in rivers and most lakes, has a salinity of 0.5 PPT or less, while the world’s oceans maintain a relatively stable average of about 35 PPT. Brackish waters exist on a gradient between these two extremes, typically ranging from 0.5 PPT up to 30 PPT. The water in Lake Pontchartrain falls into this broad brackish range, constantly fluctuating depending on the volume of freshwater input and the amount of tidal exchange with the Gulf.
The Unique Geography That Creates the Salinity
The complex salinity of Lake Pontchartrain is the direct result of two opposing geographical forces: the consistent flow of freshwater from the north and the tidal influence of saltwater from the east.
Freshwater Input
Freshwater enters the lake primarily from rivers draining the Florida Parishes to the north, including the Tangipahoa, Tchefuncte, and Tickfaw rivers, along with flow from the adjacent Lake Maurepas via Pass Manchac. This constant influx creates a noticeable gradient, with the lake’s western and northern shores often exhibiting near-freshwater conditions.
Saltwater Intrusion
Saltwater intrusion occurs through two narrow natural channels on the eastern side: the Rigolets and the Chef Menteur Pass. These passes connect Lake Pontchartrain to Lake Borgne, which is connected to the Gulf of Mexico, allowing tidal flows to push saltier water inward. The eastern section of the lake often registers the highest salinity, sometimes reaching nearly half the concentration of open ocean water.
The Bonnet Carré Spillway
A third, intermittent force is the controlled diversion of the Mississippi River through the Bonnet Carré Spillway, a flood-control structure located on the lake’s southwestern side. When the Mississippi River reaches high flood stages, the spillway is opened to divert massive volumes of freshwater into the lake to protect New Orleans and surrounding areas. This action can dramatically and rapidly decrease the lake’s overall salinity, temporarily pushing the entire body of water closer to a freshwater state. The delicate balance of the lake’s salinity is therefore a constant negotiation between natural river discharge, Gulf tides, and human-engineered flood control.
Ecosystem Adaptations and Water Use
The fluctuating salinity of Lake Pontchartrain requires its ecosystem to be highly adaptable, supporting a unique blend of organisms that can tolerate a wide range of salt concentrations. This environment functions as an important nursery ground for many species.
These include:
- Marine animals like white shrimp, blue crabs, southern flounder, and spotted seatrout.
- Fish associated with freshwater, such as the freshwater drum.
- Highly adaptable species like the bull shark, which can easily migrate between saltwater and freshwater environments.
When the Bonnet Carré Spillway is opened, the sudden influx of freshwater and sediment can cause temporary short-term stress, particularly for less mobile, salt-sensitive organisms like oysters. Despite these short-term disruptions, the long-range effect of the river diversion is generally beneficial, as it introduces valuable nutrients into the system. This nutrient input is associated with increased fisheries production for crabs and other species in the years following a flood event. The lake serves the surrounding urban population for recreation, including boating, fishing, and swimming, making water quality a perennial public concern. The primary water quality challenge stems from non-point source pollution, where stormwater runoff from urban areas carries contaminants and pathogens into the estuarine system.