A fundamental understanding of flow within single, or discrete, fractures is necessary so that fluid flow and chemical transport in fractured rocks can be accurately characterized. To explore discrete fracture flow, we conducted a series of experiments to study the movement of water through four artificial fractures, each with a different two-dimensional surface topography. We then performed fluid flow simulations using a lattice gas automata (LGA) numerical model to compare with the experimental results and to obtain detailed pictures of the fluid velocity fields within these fractures. The flow experiments and LGA simulations, all performed under fully saturated and laminar flow conditions, used fracture aperture values ranging from 0.25 to 1.80 mm. Our main focus was to determine how the cubic law, derived for fluid flow through parallel plates, could be modified to accommodate a tortuous fracture geometry. Our experimental and numerical results led to the proposal of a new conceptual model. This model states that when the aperture is measured normal to the flow path and is harmonically averaged and when the tortuosity of the flow path is included in the calculation of the pressure gradient, the cubic law can adequately predict the flow rate through fractures with a nonparallel geometry. Our study has implications for interpreting laboratory fracture flow data and for improving predictive numerical models. ¿ 1999 American Geophysical Union