Results are presented from a numerical simulation of three-dimensional ¯ow hydraulics around a mid-channel bar carried out using the FLUENT/UNS computational ¯uid dynamics (CFD) software package. FLUENT/ UNS solves the three-dimensional Reynolds-averaged form of the Navier±Stokes equations. Turbulence closure is achieved using a RNG k±e model. Simulated ¯ow velocities are compared with measured twodimensional velocities (downstream and cross-stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the ¯ow structures observed in the ®eld. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r 0Á88) and a mean dierence of 0 . 09 m s À1 . Deviations between simulated and measured velocities may be identi®ed that are both random and systematic. The former may re¯ect a number of factors including subgrid-scale natural spatial variability in ¯ow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh con®guration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic dierences between simulated and measured ¯ow velocities. These results illustrate the potential for using CFD software to simulate ¯ow hydraulics in natural channels with complex con®gurations. They also highlight the need for detailed spatially distributed datasets of three-dimensional ¯ow variables to establish the accuracy and applicability of CFD software.