NEWS NOTES ON SUSTAINABLE WATER RESOURCESMobile Bay"Incised valleys along the Gulf coast commonly resul...

Mobile Bay

"Incised valleys along the Gulf coast commonly result from rivers eroding rapidly in response to a fall in sea level. As sea level rises, sediments fill incised valleys and form nearshore elongated sandbodies such as barrier islands. These sandbodies can be potential sites for hard-mineral accumulations and are modern analogues to buried sands in the ancient rock record with high potential of being oil and gas reservoirs. Processes that formed residual sediment accumulations may also help to predict the outcome of man's erosion mitigation and wetland nourishment efforts. Today, the geologic imprint of incised valleys across the continental shelf provides evidence of sea-level change over the past 18,000 years."

- Jack Kindinger, U.S. Geological Survey

“Mobile Bay and estuaries along the Gulf of Mexico margin typically originate as incised fluvial valleys that formed during the most recent drop in sea level, and were then drowned by the subsequent postglacial sea-level rise. Most of these estuaries have been filling with sediment from fluvial and marine sources. The Mississippi-Alabama shelf province is defined by characteristics resulting from deltaic deposition advancing and receding as sea level rose and fell. A good understanding of the geologic processes that govern the distribution of sediments in this environment will allow predictions about the possible outcomes of man's efforts to mitigate shoreline erosion.”

“Some 15,000 to 18,000 years ago, sea level was about 300 feet lower than today and the coastline was near the edge of the continental shelf; that is, the abyssal deep was only a short distance offshore. 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.”