Laboratory experiments of flow through epoxy replicas of intersecting natural fractures show that solute transport deviates significantly from predictions of two-dimensional streamline routing through parallel plate intersections. Surface roughness in intersecting fractures causes two major discrepancies between mixing experiments and the parallel plate predictions. First, the fluids exiting the intersection are not uniformly mixed, but consist of ribbons with varied solute concentrations which tend to follow streamlines in the flow. Some of these streams maintain a nearly pure inlet fluid composition as they exit the intersection and traverse the outlet fracture. Second, when the outlet fluids are collected and homogenized, we find that this complex redirection of streamlines within natural rough-walled fracture intersections results in more total mixing than is predicted by the parallel plate model. ¿ 2001 American Geophysical Union