The Espaņola Basin contains the principal aquifer supplying drinking water to a rapidly growing population in Northern New Mexico. Although geologic complexity in the basin is well documented, the degree to which geologic features control groundwater flow is unclear. We present an analysis ranking alternative geologic models based on their ability to represent existing groundwater data and associated impacts on groundwater transport predictions. The geometry of key geologic features, with associated uncertainty, is expressed via alternative 3-D geologic models. Using non-linear optimization, we estimate medium properties for each of alternative geologic models based on the large dataset of stream-baseflow flux and spatially- and temporally-varying water-level measurements throughout the basin. Using statistical methods, we rank the models based on their ability to represent the measurements and to impact groundwater transport predictions. In areas where water level data are sufficiently dense, this process objectively identifies those geologic features that have hydrologic significance and those features that are clearly "flow neutral". It also identifies the hydrostratigraphic structures which are import to our transport predictions.