Abstract: The Anatomy and Physiology of a Gulf Coast Delta

W. C. Isphording, R. B. Jackson, F. D. Imsand

Deltas are often perceived as final resting sites of the majority of sediments carried into these systems. As such, most deltas are believed to possess significant sediment trap efficiencies. In the northern Gulf of Mexico basin large delta complexes are few. From Texas, eastward to the Florida Keys, only three major deltas are present. These include the delta of the Mississippi River, that of the Mobile River, and the Apalachicola River delta. The latter, because of numerous dams upstream, has been seriously affected by man and therefore is less than ideal for the study of deltaic processes; the Mississippi Delta, similarly, has been markedly affected by any flood control projects whose effect must be estimated when attempting to interpret processes now taking place in t is system. The Mobile River Delta, in contrast has escaped major anthropogenic effects and enjoys a number of features that are apparently unique among large North American delta complexes.

The Mobile River delta serves as the terminus for the Nation's fourth largest river system, in terms of annual discharge, and is exceeded only by the Mississippi, Columbia, and Yukon. Unlike the others, however, the distributary river system discharging from this delta does not empty into an ocean, but rather flows into estuary having a restricted opening to the Gulf of Mexico (Mobile Bay). Accurate measurements have been made of the quantities and types of sediment that enter this delta, are trapped by the delta, and which pass through the delta. Some 33 percent of the total sediment entering the delta complex becomes trapped, with the remainder passing through to Mobile Bay. The bay, in turn, traps an additional 52 percent. Thus, only 16 percent of the original sediment load is disc arged to the Gulf of Mexico. The trap efficiency, and composition of the delta sediments, have important consequences with respect to contaminants derived from upstream municipal and industrial sources. The plus side is that Alabama's largest wetland region has been minimally effected by these contaminants because they largely pass completely through the delta; the bad news is that the sediments of Mobile Bay therefore receive the bulk of this load and have borne the major impact.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas