The Mississippi River is overwhelmingly a freshwater system. This colossal North American river flows from its northern source as a massive channel of land-based runoff and precipitation, maintaining its freshwater identity for the vast majority of its length. The only exception to this rule occurs at the river’s very end, where its immense flow meets the Gulf of Mexico. This mixing zone creates a dynamic, localized environment where the water quality shifts dramatically, though the main body of the river remains fresh.
The Primary Identity: Why the Mississippi is Freshwater
The Mississippi River begins its journey at Lake Itasca in northern Minnesota, starting as a narrow, clear stream fed entirely by rainfall, snowmelt, and groundwater. This initial water source contains only minimal dissolved solids, establishing its freshwater status from the start. Over its approximately 2,340-mile course south to the Gulf of Mexico, the river collects water from one of the world’s largest drainage basins.
This immense watershed covers roughly 40% of the contiguous United States, drawing in water from 31 states and two Canadian provinces through major tributaries like the Missouri and Ohio Rivers. The sheer volume of water collected from this landmass—a flow that averages about 600,000 cubic feet per second near its mouth—is what guarantees the river’s freshwater composition. This constant, enormous influx of terrestrial water pushes back against any potential oceanic influence for thousands of miles. The water that enters the Gulf of Mexico carries dissolved minerals.
The Salinity Shift: Understanding the Delta and Estuary
The interaction between the river and the ocean takes place in the Mississippi River Delta, a large area near the Louisiana coast that functions as a classic estuarine environment. An estuary is a semi-enclosed body of water where freshwater from rivers mixes with saline seawater. This process creates a gradient of water types, ranging from fresh to brackish to fully saline.
The most specific factor influencing this area is the “salt wedge,” a phenomenon driven by density differences between the two water masses. Ocean water from the Gulf of Mexico is significantly denser than the lighter river water. When the two meet, the Gulf’s saline water sinks and moves upstream along the riverbed, creating a layer of salt water beneath the outgoing freshwater flow. This wedge can extend many miles upriver, especially during periods of drought when the Mississippi’s flow is low.
During extreme low-flow events, the salt wedge has been known to threaten drinking water intakes far upstream, sometimes reaching areas near New Orleans. This temporary saltwater intrusion affects the water quality at the bottom of the channel and requires human intervention, such as the construction of underwater sills, to slow its progression. Nevertheless, the main, upper layer of the river remains freshwater flowing toward the sea, meaning the river is only partially and temporarily saline in the deepest parts of its lowest reaches.
Measuring the Difference: Defining Salinity
Salinity is the scientific measure of the total concentration of dissolved salts in a body of water. Scientists use units like Practical Salinity Units (PSU), which is a dimensionless measurement based on water conductivity, or parts per thousand (ppt). Seawater is defined by a high salinity, typically measuring around 35 PSU, which is equivalent to 35 grams of salt dissolved per kilogram of water.
Water is officially classified as freshwater when its salt content is less than 0.5 PSU. Brackish water, the mixture found in estuaries, falls into the range between 0.5 PSU and 30 PSU, while anything above 30 PSU is considered saline. The vast majority of the Mississippi River tests well below the 0.5 PSU mark, firmly placing it in the freshwater category. Even at the delta, the surface layer maintains a very low salinity, only becoming brackish or saline in the deeper, lower layers where the dense Gulf water intrudes.