The recent geomorphologic history of the Apalachicola River can be related to local changes in sea level and the sediment output of the river. These changes can be observed in seismic data and boreholes collected in the bay. This project employed seismic, borehole and geochronologic data in order to map and better understand the paleogeography of the Apalachicola River and Bay region. These maps include; the seabed, the top of a mid-Holocene stratigraphic unit, the top of the Pleistocene, the top of the uppermost Mio-Pliocene unit, which includes a karst surface. In addition, Holocene-aged fluvial paleo-channels and paleo-deltas were identified in the subsurface and their paleogeographic relationships were mapped. The uppermost Mio-Pliocene aged sedimentary unit was described by Schnable (1966) on the basis of borehole data, as primarily sandy and clayey limestones, known as the Choctawhatchee Formation (Schnable, 1966). Huddlestun (1976) later described the same unit as the Intracoastal Formation, due to its location in the Intracoastal Waterway. Schmidt (1984) described the Intracoastal Formation as an easily recognizable stratigraphic unit composed of sandy, highly microfossliferous calcarenitic limestone. The thickness and dip of the Intracoastal Formation varies widely over the Apalachicola River region, due to the influence of the regionsís largest subsurface structure, the Apalachicola Embayment (Schmidt, 1984). The embayment dates back to the early Tertiary and is the result of regional tectonic forces. The embayment is located between the Chatahoochee Anticline to the west and the Ocala Platform to the east (Rupert, 1997). The northern end of the Apalachicola Embayment narrows and extends into the Gulf Trough in southern Georgia (Rupert, 1997). It widens and deepens as it extends into the modern Gulf of Mexico (Rupert, 1997). The southernmost extent of the embayment is not well established due to the lack of offshore well data (Schmidt, 1984).
The early to mid-Holocene paleo-deltaic features beneath the modern estuary migrated from west to east over the last approximately 7000 years. The specific subsurface features were identified and located by use of both seismic data and vibrocore logs. The depth of the features observed in the seismic data was calibrated based on bridge borehole transects collected from the Eastpoint to Apalachicola and the Apalachicola to St George Island bridge-causeways.
The Apalachicola Riverís paleo-discharge was determined by using the Manning equation to calculate the maximum discharge of the river, based on the channel geometry and regional gradient. Several paleochannel profiles were found and measured. The calculated paleo-discharge was 86,000 cfs. This discharge was based on the entire cross-section of the river and therefore represents a bank-full or maximum discharge. The modern and paleo-streamflow were found to be comparable. The calculated paleo-discharge value falls within the range of the average annual peak discharge of the modern Apalachicola over the last 30 years. A possible explanation comes from Holocene climate data extracted from the palynology of long cores from regional lakes. Data from two lakes indicates that climate in the region has changed little in the past 6,000 years and perhaps for as much as 8,000 years. The paleofluvial history of the regionís largest river, the Apalachicola, appears to corroborate that finding.