STREAM/AQUIFER INTERACTIONS IN A KARSTIC RIVER BASIN, ALAPAHA RIVER, FLORIDA

The Alapaha River originates in southern Georgia and flows south into northern Florida, where it discharges into the Suwannee River. During low to moderate flows in the river, surface discharge is captured in part or entirely by sinkholes and swallets in the bed of the river shortly after the river enters north Florida. Surface flow in the Alapaha reaches the Suwannee River only during high flows and floods. Captured surface-water travels through conduits in the aquifer and eventually discharges through the Alapaha Rise into the Suwannee River. During large floods, the conduit system is overwhelmed and significant amounts of surface-water are taken up as storage in the matrix of the aquifer. The current available flow data for the Alapaha River in Florida is insufficient for a detailed analysis of the stream/aquifer interactions. However, through regression analysis an ~70-year period of record was simulated for the gauges located upstream and downstream of the sinks in the Alapaha River. To our knowledge, this study represents the first time this approach has been used to quantify the relationship between surface and groundwater flow in this type of system. Comparison of discharge upstream and downstream of the sinks located in the streambed indicates the amount of Alapaha River flow that enters the groundwater system. When upstream discharge is below ~500 cfs, all surface-water flow is diverted into the aquifer. When upstream discharge is greater than ~500 cfs, about 20% of the flow above 500 cfs is also captured. Flow into the aquifer exceeds downstream flow approximately two thirds of the time. The contemporaneous data for the discharge upstream from the sinks and the Alapaha Rise display a weak relationship during low flows and no relationship during high flows. However, reasonably good correlations exist for Alapaha Rise discharge lagged 6 and 21 days behind the upstream discharge for the low and high flow data, respectively. An even better relationship is found between Alapaha Rise discharge data and the water level data for a nearby shallow well. An analysis of the relationship between Alapaha Rise discharge and Suwannee River stage indicates high water in the Suwannee River has little effect on discharge from the Alapaha Rise, probably due to the long lag times inherent to this system.