Mobile Bay is fed by six different rivers – most importantly the Mobile and the Tensaw – making it the fourth largest estuary in the United States. During wet months, the influence of freshwater flowing out of the estuary can be measured as far as 50 miles out into the Gulf. During drier times, salt water from the Gulf can reach 80 miles upstream, bringing saltwater species with it.
The Mobile-Tensaw Delta is at the northern crown of the estuary, and benefits from hundreds of thousands of acres of bottomland forests in its watershed that are flooded for months at a time. As a result, the delta and the bay are recognized nationally for their biodiversity.
Mobile Bay is the only place in the world known to have a “jubilee” – a rare combination of temperatures, tides and winds resulting in a low-oxygen event that forces fish and crustaceans to the surface of the water near the shoreline. Jubilees are natural occurrences but human changes to the coast may have increased the frequency of low-dissolved-oxygen events. However, for the early residents of Alabama, having fish and shellfish so accessible and easy to catch was a cause for celebration, hence the name “jubilee.”
Mobile Bay faces many serious environmental challenges. More than half of Alabama’s coastal wetlands were lost by 1980. Storms, saltwater intrusion, sea level rise and continued human activities mean that the bay is still losing wetlands at a rapid rate. The bay now has less than a third of the seagrass beds it did in 1940. Oyster reefs have declined significantly as well, but a lack of baseline data makes it difficult to understand exactly how much. Urbanization, altered hydrology and invasive species have harmed water quality, altered salinities, increased sediment loads and reduced biodiversity.