Mobile Bay, a prominent body of water along the Gulf Coast, often prompts questions about its water composition.
Mobile Bay’s Water Characteristics
Mobile Bay is primarily a brackish estuary, a mixture of freshwater and saltwater. Brackish water contains an intermediate level of salinity, typically ranging from 0.5 to 30 parts per thousand (ppt), saltier than freshwater but less salty than seawater. This transitional nature is a defining feature of estuaries.
Salinity levels within Mobile Bay are not uniform; they fluctuate significantly across different areas and throughout the year. Waters closer to the Gulf of Mexico tend to exhibit higher salinity, while areas nearer to river mouths have lower salinity concentrations. Seasonal variations also occur, with salinity generally lower during winter and spring due to increased river discharge and higher during summer and fall when freshwater inflow decreases. The bay’s average depth is approximately 10 feet (3 meters), though shipping channels can reach depths exceeding 75 feet (23 meters). This physical characteristic contributes to the dynamic mixing patterns.
Influences on Mobile Bay’s Salinity
Mobile Bay’s salinity is shaped by freshwater inflows from a vast river system and saltwater intrusions from the Gulf of Mexico. The Mobile River system, which includes the Mobile, Tombigbee-Black Warrior, and Alabama-Coosa-Tallapoosa Rivers, is the primary source of freshwater, contributing over 95% of the total freshwater entering the bay. This system represents the fourth largest in the United States by streamflow. Freshwater discharge from these rivers can range significantly, from approximately 60,000 to 3,700,000 gallons per second, directly affecting the bay’s overall salinity.
Saltwater from the Gulf of Mexico enters Mobile Bay primarily through tidal exchanges, with about 85% of this water exchange occurring through Main Pass Sound. Tides in the bay are predominantly diurnal (one high and one low tide per day), driving saline water inflow. The bay’s broad, shallow, and triangular shape also influences how freshwater and saltwater mix. Deeper shipping channels, such as the Main Ship Channel, act as conduits, allowing more saline Gulf waters to penetrate further inland and sometimes form a distinct layer of denser saltwater beneath fresher surface water. Additionally, human-made structures like the Mobile Bay Causeway can alter flow patterns, and wind events can further influence the distribution of salinity throughout the bay.