Goodenough Spring is a sub-lacustrine spring situated 43 meters beneath the surface of the international Amistad Reservoir on the Rio Grande. Its source is the Edwards-Trinity aquifer and prior to its inundation in 1968, it was a very high volume and high quality tributary to the Rio Grande. Since 1968, Goodenough Spring has been mixing with the more saline water of the reservoir. However, the mixing is not uniform, leading to horizontal lenses of spring water within the strongly stratified water column during the summer and surface infusion of spring water in the winter. Spring and fall are transitional periods. From a moving boat, we have tracked winter spring flows using simultaneous 1 Hz measurements of temperature, turbidity, in vivo chlorophyll fluorescence, conductivity, pH and GPS readings. This sampling rate yields a 10 m resolution along the transects. Subsurface water column structure was inferred from spot profiles of these same parameters reading at 1 meter depth intervals at 15 locations. Goodenough Spring carries a significant NO3-N load and discharges it into the reservoir at a point far removed from other major nitrogen inputs. Water samples were also collected along the transects for subsequent NO3-N analysis. Our data indicate that Goodenough Spring is a significant contributor to the quality and quantity of water in Amistad Reservoir.