As part of a U.S. Geological Survey Groundwater Resources Program study, a three-dimensional geologic model was constructed for approximately 53,000 square miles of the Columbia Plateau in Idaho, Oregon, and Washington. This model was constructed to define the general aquifer system geometry for use in a regional numerical groundwater flow model. Simplifications and assumptions consistent with this ultimate goal and with the uncertainty in the available data were made. The model units consist of Miocene-age Columbia River Basalt Group strata and younger sedimentary overburden covering approximately 44,000 square miles. Data were compiled from numerous databases and detailed studies that were completed during the past three decades. These data include stratigraphic interpretations of more than 13,000 wells and a contiguous compilation of surficial geology and structural features in the study area. These data were simplified and used to construct piecewise-smooth trend surfaces that represent upper and lower subsurface model unit boundaries in this complex folded and faulted terrain. The smoothness of the surfaces implicitly represents uncertainty in prediction of each surface resulting from data gaps, errors in borehole interpretations, errors in mapped contact locations, and uncertainty in the shape of the paleosurface upon which flood basalts were emplaced. Surfaces were recombined using a rule-based algorithm to construct a fully three-dimensional model with a 500-foot grid resolution that is consistent with the data and for which error estimates may be made. The modeling process yielded improved estimates of unit volumes, refinement of location of large structural features, and identification of features that may be important for ongoing groundwater studies.