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MOBILE BAY NATIONAL ESTUARY PROGRAM COMPREHENSIVE CONSERVATION AND MANAGEMENT PLAN
Citation:
MOBILE BAY NATIONAL ESTUARY PROGRAM COMPREHENSIVE CONSERVATION AND MANAGEMENT PLAN. U.S. Environmental Protection Agency, Washington, D.C., 2002.
Description:
In simplest terms, an estuary is defined as an area where rivers meet the sea. They are transitional zones where freshwater rivers meet tidally influenced marine waters. Estuaries are considered environmentally and economically important because of their exceptional biological diversity and productivity. In fact, the whole of an estuary is truly greater than the sum of its parts. These areas support both fresh and saltwater plant and animal species and serve as nursery habitat for many commercially important seafood species.
The Mobile Bay Estuary is the coastal transition zone between the Mobile Bay watershed and the Gulf of Mexico. The Mobile Bay watershed the land area that drains into the Bay covers two thirds of the state of Alabama and portions of Mississippi, Georgia, and Tennessee. It is the fourth largest watershed in the United States in terms of flow volume, and is the sixth largest river system in area in the United States. The Mobile Bay and Delta are subject to an unusually large number of major uses with national implications, including the Tennessee-Tombigbee Waterway, the Port of Alabama, commercial fisheries, industry, tourism and recreation, and coastal development.
When the Mobile Bay NEP was first established, the Management Conference chose to limit the Program's management focus to just the portions of the Mobile Bay watershed that lie within Baldwin and Mobile Counties. Specifically, the program boundary includes the portions of the two counties that drain into Mobile Bay, as well as the Mississippi Sound westward to the Alabama- Mississippi state line, and the Alabama state marine waters in the north-central Gulf of Mexico, extending three miles south of Dauphin Island and the Fort Morgan peninsula. The northern boundary does not represent any biological or geophysical lines of distinction. From the beginning, however, the Mobile Bay NEP has been cognizant of the role that the rest of the much larger Mobile Basin plays in management decisions within the NEP study area. For example, the northern reaches of the larger watershed area contribute directly to the volume of freshwater that enters the estuary, which, in turn, affects the salinity dynamics of Mobile Bay.
Water runs downhill, and a raindrop that falls in northwest Georgia can, theoretically, eventually enter Mobile Bay, which sits at the bottom of that hill. During periods of intense regional rainfall, a large volume of water can enter Mobile Bay at once, carrying with it sediments, nutrients, and chemicals gathered upstream. Mobile Bay serves as the natural receiving basin for this upstream flow. In addition, as the freshwater reaches sea level at Mobile Bay, the flow slows dramatically, allowing suspended sediments to settle out into the Bay. As a result, Mobile Bay has developed as a naturally shallow bay.
The Bay itself is approximately 32 miles 'tall' (north to south), 23 miles wide at its widest point, and about 10 miles wide where it meets the Mobile-Tensaw River Delta. The average Bay depth is approximately 10 feet, which is very shallow for a bay of this size.
Mobile Bay's salinity is driven predominantly by freshwater inflow from the large Mobile Bay watershed. A combination of winds and tidal regimes delivers salty Gulf waters into the Bay from the south. As a result, salinity in Mobile Bay is highly dynamic.
The climate of the Mobile Bay area is characterized by warm, humid summers and relatively mild winters. Average maximum summer temperatures vary from the high 80s to low 90s Fahrenheit, with a record high temperature of 105oF set in 2000. During winter months, freezing is not uncommon, and temperatures less than 19oF occur every other year, on average. Summer thunderstorms and winter cold fronts produce heavy downpours and contribute to an average of about 65 inches per year of rainfall the highest average for any U.S. city of Mobile's size. The central Gulf Coast also has one of the highest frequencies of hurricane landfalls in the nation. The Bay is additionally influenced by tidal changes that average a little less than 1= feet throughout the year, with a maximum tidal change of a little over 2= feet. All of these factors, combined with highly variable river flows, contribute to a hydrology that is dynamic, complex, and necessary to support the variety of plants and animals existing in the Mobile Bay Estuary.