The Apalachicola River flows unimpeded for 106 miles from Jim Woodruff Lock and Dam to the Gulf of Mexico. The river drains about 2,600 sq. miles and its shallow estuary covers about 208 sq. miles. Tidal influences do not extend beyond 25 miles upstream from the river's mouth. The Apalachicola River falls 40 feet as it flows through the Gulf Coast Lowlands. The width of the river ranges from several hundred feet when confined to its banks to nearly 4-1/2 miles during high flows. The discharge of the Apalachicola River is 21st in magnitude among the rivers of the conterminous United States, and is the largest in Florida, accounting for 35 percent of fresh water flow on the western coast of Florida (Livingston, 1992). During 1977-92, the discharge of the Apalachicola River based on mean daily discharge at Sumatra, Florida, was 19,602 cfs. Mean daily discharge at Sumatra ranged from 5,800 cfs in 1981 to 178,000 cfs in 1990. Eighty percent of the Apalachicola River flow is contributed by the Chattahoochee and Flint Rivers, 11 percent from the Chipola River, and less than 10 percent from ground water and overland flow (Elder and others, 1988). The Chipola River-Apalachicola River's largest tributary-drains one-half of the Apalachicola River basin. The Chipola River is classified as a spring-fed river with baseflow derived principally from aquifers.
Because of rainfall-distribution patterns, the average annual runoff from the Chattahoochee River exceeds that of the Flint River. The Chattahoochee River makes a greater contribution to peak flows in the Apalachicola River than the Flint River. However, during extreme dry periods, the greater flow contribution in the Apalachicola River comes from the Flint River, where baseflow is sustained by ground-water discharges (Elder and others, 1988).
Leitman and others (1983) studied stage and discharge records from 1929-79 to determine if significant hydrologic changes occurred in the Apalachicola River as a result of dam-flow regulation. Dams have had little effect on the magnitude of high flows or seasonal distribution of discharge over an annual cycle. Dam regulation did reduce the amount of time that flow was at low extremes. Water stages in the river within the first 30 miles downstream of Jim Woodruff Lock and Dam have lowered due to scouring of the river bottom.