To observe flow channeling in situ, surface ground-penetrating radar (GPR) was used to detect a saline tracer moving through a sub-horizontal bedrock fracture. The tracer is mapped using amplitude variations resulting from the significant difference in electromagnetic properties between the tracer and natural ground water within the fracture. Results show that hydrogeophysical investigations using GPR can successfully image tracer movement in a variety of flow configurations in three spatial dimensions through time (hence 4D). Observed tracer travel paths demonstrated meter-scale spatial variability that may be attributed to heterogeneity in fracture aperture.