Recently, it has been found that several species of oaks (e.g. Q. douglasii) are phreatophytes, or predominately use groundwater. The objectives of our study were to determine if riparian bur oak (Quercus macrocarpa) also used groundwater. This study took place in Homestead National Monument (Homestead, NM), Nebraska, where bur oak grow within the historical floodplain of Cub Creek with groundwater six to seven meters below the surface. Plant and soil samples for isotopic analysis were collected from three sites during 2008 and 2009. Water table depth was measured continuously, and river stage and precipitation were monitored daily by Homestead NM staff. We analyzed river water, groundwater, twig xylem water, and soil water for natural abundance of oxygen isotopes (which can be used as a natural tracer to identify different sources of water). A three-pool mixing model indicated that bur oak were using from 70% to 88% groundwater, 10% to 12% deep soil water, and a maximum of only 8% from shallow soil water. This, in conjunction with diel groundwater fluctuations, indicates that these riparian bur oak are likely phreatophytic and are using groundwater as their predominant source of water.