Mobile Bay's average depth is only about 10 feet, which is among the shallowest for a bay of its size. It is approximately 32 miles long from the Mobile-Tensaw Delta to its mouth, 23 miles across at its widest point, and about 10 miles across at the City of Mobile. A combination of wind and tide delivers salty Gulf waters from the south into the Bay that mix with varying amounts of freshwater from the Mobile-Tensaw River Delta. Due to the shallow nature of the Bay, dynamic climatic conditions, and man-made hydrologic modifications, salinity conditions in the Bay are remarkably variable.
Several factors contribute to the ever-changing salinity conditions which characterize Mobile Bay. Summer thunderstorms and winter cold fronts produce heavy downpours contributing to an average of 66 inches of rainfall annually, the highest average nationally for a city the size of Mobile, along with a relatively high frequency of tropical cyclone landfalls. Consequently, river flows are naturally highly variable. The Bay is influenced by a single daily or "diurnal" tidal cycle with tide changes averaging a little less than a foot and a half feet. The resulting hydrology is dynamic, complex, and necessary to support the diversity of plant and animals found in and around the Bay.
The Alabama State Port Authority (ASPA), established in 1928 as a state governmental agency, operates the deepwater port facilities in Mobile Bay. The port complex includes facilities for handling general cargo, such as containers, forest products, and metals, as well as liquid bulk and dry bulk cargo, such as chemicals, coal, iron ore, and steel. The port complex features more than four million square feet of warehouse space and open yards and almost 40 berths for ships. A 75-mile rail line links Port of Mobile facilities and provides connections to major freight railroads. In 2013, the Port of Mobile ranked 9th, out of 171 U.S. ports in total metric tons of U.S. exports, 17th in total metric tons of U.S. imports, and 13th in total U.S. volume of trade.