Field data-based finite-element simulations of flow partitioning between fractures and a permeable rock matrix reveal critical fracture aperture values that mark the transition from matrix to fracture dominated flow. For matrix permeabilities of 0.00 1--1 D, the matrix either dominates or contributes significantly to the total flow. The percentage of the flow-normal cross-section that is occupied by fractures, Af, strongly influences the fracture-matrix permeability ratio, above which fractures will dominate flow. This ratio is 102--104 for Af = 10-4--10-3 (mean = 5 ¿ 10-3), but also depends on the proportion of fractures which fully penetrate the representative elementary volume. Fluid-velocity spectra for the fractured rock have three important characteristics: (1) Darcy velocity is only poorly correlated with permeability, (2) flow velocities have characteristic values, even if fracture-length frequency relations are self similar, and (3) fracture and matrix velocities overlap.