The geologic history of the Mobile Bay area has been dominated by rapidly changing sea levels over the past several thousand years. A seismic profile from the eastern flank of Mobile Bay, Alabama, showing the typical stratification of sea-level succession found within incised valleys of the northern Gulf of Mexico. This partial sea level cycle begins with a lowstand unconformity formed during the glacial maximum, the Mobile River meandered across the valley, and as the rising sea flooded the central basin fine-grained estuarine sediments were deposited in the bay. About 6,000 years ago, sea level rose to a level about 15 feet below its present level. Mobile Bay was only a freshwater river, the west Florida coast was about 15 miles further west than its present position, and corals in the Florida Keys began to flourish.
Sea level continues to rise, and portions of the Gulf coast are subsiding.
In addition, levees on the Mississippi River have reduced the volume of sediment reaching coastal wetlands of the Northern Gulf. The combined effect of human activity and natural forces has caused the loss of barrier islands along the Gulf coast and the consequent loss of wetlands. These losses result in billions of dollars of recreational and commercial value to the affected States. Efforts to mitigate these losses have relied on cooperative USGS studies that described the outcomes of a number of strategies. USGS scientists studying sediment accumulations on the continental shelf have provided new insights to the formation of barrier islands.