Potentiometric surface maps of the Upper Floridan aquifer show two depressions around the St. Johns River frm the city of Jacksonville south toward Green Cove Springs. These depressions, depending on their locations, are the result of withdrawals from agricultural, industrial, domestic and public-supply wells, diffuse upward leakage, and discharge from springs. Submerged springs that discharge into the St. Johns River between Jacksonville and Green Cove Springs have been thought to exist, but locating and evaluating these springs had not been attempted before this investigation. Thermal infrared imagery, seismic reflection, and numerous interviews with local residents were used to locate springs. An airborne thermal infrared survey was conducted along a section of the St. Johns River in northeastern Florida during February 1992 to detect possible sources of ground-water discharge to the river. An infrared image displayed one thermal anomaly in the St. Johns River which is associated with a previously unknown spring discharge from the Floridan aquifer system. Thermal anomalies also were observed at six locations where municipal facilities discharge treated wastewater to the river. Results of seismic reflection surveys indicate the presence of collapse and other karst features underlying the St. Johns River. These features indicate that the surficial deposits and the Hawthorn Formation that underlie the river probably do not consist of continuous beds. The collapse or deformation of the Hawthorn Formation or the presence of permeable sediment of localized extent could create zones of relatively high vertical leakance. This could provide a more direct hydraulic connection between the Upper Floridan aquifer and the river. Water samples collected from the only submerged spring in the St. Johns River within the Jacksonville-Green Cove Springs reach indicate that the source of the water is the Floridan aquifer system. Chloride and sulfate concentrations were 12 and 340 milligrams per liter, respectively. Specific conductance was 826 microsiemens per centimeter and the temperature of the water discharging from the spring was 25.1 degrees Celsius. The ratio of 87 Strontium/86 Strontium also indicates that the springwater has been in contact with rock materials of Eocene age, providing additional evidence that the springwater is derived from the Floridan aquifer system.