The rate of groundwater flow has long been recognized as a critical control on solute transport in the subsurface. However, information about groundwater flux and its variability in space is rarely available, especially at the resolution required for investigations at sites of groundwater contamination. Recently, high-resolution information about vertical variations in groundwater flux was obtained using fiber-optic distributed temperature sensing technology to monitor the temperature response to active heating in a well. A series of vertical thermal profiles were acquired at a 1.4 cm resolution in a sand and gravel aquifer. These high-resolution profiles, which display many of the same general features as hydraulic conductivity (K) profiles obtained using multiple techniques at the same well, provide new insights into site hydrostratigraphy. In particular, the near-continuous profiles reveal the existence of thin zones of relatively high or low velocity that would be difficult to detect using other methods. These profiles also demonstrate that vertical variations in K may not be an accurate indicator of vertical variability in groundwater flux in highly heterogeneous aquifers.