Using stream gage data to quantify surface water/groundwater exchanges between the Upper Floridan aquifer and the lower Flint River, Georgia, USA, 1989-2003

Historical discharge measurements from two U. S. Geological Survey (USGS) gaging stations were used to quantify surface water/groundwater exchanges between the lower Flint River and the Upper Floridan aquifer (UFA) in south Georgia. This approach, based on differences between offset-corrected (OC) downstream and upstream discharge estimates (Qdownstream(OC)-Qupstream=Qdiff), was feasible because of the unique geohydrology of this 55 kilometer (km) stream section in which numerous large spring conduits substantially supplement base flow, and tributary stream contributions are relatively minor. Qdiff values for 1989 through 2003 revealed patterns in groundwater/surface water exchanges that included streamflow losses that exceeded 150 cubic meters per second (cms) during floods with corresponding flow returns of equal magnitude as river stage declined. Field data from temperature sensors placed inside large springs demonstrated flow reversals that were identified by negative Qdiff values. After adjustments were made to account for small tributary creek inputs, estimates of groundwater contributions to this stream section ranged from 6 to 22 cms with a mean of 13 cms when averaged over the study period. The highest groundwater discharge to the stream occurred during the spring when regional groundwater levels peak following heavy winter/spring rains and corresponding rates of evapotranspiration are low. During extreme periods of drought, groundwater contributions to the lower Flint River greatly declined.Sponsored by: Georgia Environmental Protection Division U.S. Geological Survey, Georgia Water Science Center U.S. Department of Agriculture, Natural Resources Conservation Service Georgia Institute of Technology, Georgia Water Resources Institute The University of Georgia, Water Resources Facult