Subsurface flow is primarily controlled by the distribution of hydrogeological properties. Spatially correlated random fields, currently the primary means of describing heterogeneity in these properties, underutilizes the existing knowledge of geological systems. A new computer code, the Braided Channel Simulator (BCS-3D), is used to geometrically simulate the spatial heterogeneity of hydrogeological properties using representations of surface topography to estimate the three-dimensional arrangement of subsurface units. Four methods of assigning conductivity were used to generate conductivity fields as a basis for saturated flow simulation. Variograms measured for each of the conductivity fields give no indication of underlying discrete structure. However, particle-tracking simulations showed that flow occurred along paths of preferentially high hydraulic conductivity. Results indicate that in systems where the contrast in conductivity is sufficiently great, the location and magnitude of flow is constrained by discrete internal structure. Additionally, geometrical simulation techniques such as BCS-3D can be used to produce property fields that embody these discrete geological structures. ¿ American Geophysical Union 1996