The historical and environmental forces involved in determining the geographic location of plant hybrid zones have long been of interest. Often hybrid zones appear to be intimately tied to the environment, yet because many abiotic factors covary it is often difficult to understand which are truly the most important in maintaining the position and structure of a hybrid zone. This study uses empirical data and modeling analyses to examine whether abiotic factors are responsible for the location and structure of an oak (Quercus) hybrid zone and, if so, which factors are the most important. Specifically, leaf trait measurements and ecological niche models were used to test the hypothesis that water availability plays a primary role in promoting and maintaining the location of hybridization between 2 species. Leaf trait analyses and ecological niche models both supported the hypothesis that water availability determines the location and structure of the hybrid zone. These findings lend support for the general importance of environmental factors in determining hybrid zone location and structure. Furthermore, they demonstrate how functional trait analyses and predictive ecological niche models can be used in future hybrid zone research.