HISTORIC CHANGES IN SEDIMENT DEPOSITION IN A GULF COAST ESTUARY

Apalachicola Bay, Florida is the terminus of the State's largest river system. Economically, the bay is of considerable value and provides over 90 percent of oysters harvested annually in Florida waters.. As such, any changes in sediment depositional patters are important, whether the result of natural processes or those abetted by man. Bottom sediment maps compiled from recent and historic data over the past 150 years reveal that the bay is a dynamic system and that some areas have changed repeatedly while others have remained apparently unchanged. Hurricanes have significantly altered the bay's bottom sediments on at least several occasions, both by scour and fill and as a result of the breaching of the fringing barrier island (St. George Island). Equally striking are changes that have been brought about by construction of dams in the watershed. Initially, dam construction resulted in a decrease in the coarse sediment load provided to the bay because of trapping behind the dams. Subsequently, erosion below the lowermost dam has caused accelerated widening of the flood plain of the Apalachicola River and an increase in the coarse sediment load carried into the bay. The primary cause of this can be traced to the fact that surficial strata exposed below the dam largely consist of Neogene and Quaternary coarse clastic units which are easily eroded. While changes to a bay's sediment regimen are often viewed as non-beneficial, those of a non-anthropogenic natural have actually been helpful. Hurricanes have acted to scour the bay and remove decades of sediment deposition and have also significantly reduced levels of anthropogenic contaminants in the bay. Similar effects have also been noted in other Gulf Coast estuaries.