ABSTRACT

Nearly two dozen estuaries fringe the southern margin of the United States in the Gulf of Mexico region. These estuaries annually receive approximately 50% of the nation's total runoff and, as such, serve as depositional basins for some 65 million tons of sediment each year. Although it is widely believed that the ultimate fate of an estuary is to become filled in with sediment, when depositional rate data from some Gulf estuaries are examined it becomes apparent that either (1) present-day rates of infilling differ markedly from those in the past, (2) the basic concept that estuaries generally do not equilibrate with relative sea-level rise and sediment influx is incorrect, or (3) natural events periodically occur that cause extensive erosion and scouring of the estuaries, thereby prolonging their existence.

Examination of sediment data from two of the larger Gulf estuaries clearly shows that these features may well persist for time intervals that are considerably longer than would be predicted by their annual infilling rates. Evidence from a number of estuaries strongly indicates that subsidence and sediment accumulation have reached near equilibrium in perhaps a majority of cases. Furthermore, evidence is present in some of the bays that indicates that major storms have removed significant quantities of sediment, thereby substantially extending the life of these water bodies. Because of the proclivity for such storm events in the Gulf of Mexico region, it is likely that many estuaries have been impacted by similar events or will be thus impacted in the future. Hence, prediction of the longevity of a Gulf Coast estuary from accumulation rates (and even those rates supported by radiocarbon dating) can produce erroneous results because in the space of a few hours a single storm event can remove a volume of sediment that was deposited over a time interval of hundreds, or even thousands, of years. Careful examination of sediment data from core samples can often provide evidence that removal of a portion of the sediment record has taken place. If such is the case, any attempt to establish a long-term depositional rate for the estuary is likely doomed to failure.