The focus in this works lay on the impact of Airborne Bathymetric Laser Scanning (ALB) geometry information on the simulation of in stream hydraulics in Mountain Rivers. High-resolution topographies of two mountain rivers beds (the rivers Ahr and Sill) from ALB were used. Two ways in which the ALB data was suspected to be beneficial for hydraulic calculations were assessed: i) whether a geometrical roughness parameter could be derived from the ALB information, that could then, without further calibration, be used in flow resistance equations and Reynolds Averaged Navier Stokes (RANS) 2D and 3D numerical simulations, and ii) whether in RANS simulations on the, ALB based, detailed representations of the river geometries, flow velocities and water levels were accurately reproduced, and whether this could be achieved without a roughness parameterization in the models. It was assumed that the roughness parameterization could become obsolete, when in the models, the roughness geometry was directly represented. For this purpose, the rivers hydraulics were studied with extensive field measurements of 3D flow velocities, at two sites at the Ahr and one at the Sill. The velocity data were collected with ADCP and ADV acoustic devices. They were compared to results of flow resistance equations, 2D and 3D RANS simulations, that were based on the ALB information. RANS simulations were performed with four different softwares (Hydro-AS 2D, Basement-Baseplane 2D and Telemac 2D and 3D). It was found that i) at Mountain River sites a flow resistance parameter cannot be derived from solely geometrical information, ii) the roughness could not be directly represented in RANS simulations, iii) 2D RANS calculations reach their limits when trying to accurately reproduce Mountain River in-stream velocities, and iiii) 3D RANS calculations, at least calculated on a discretization scale of 0.5 m, also reached their limits. An alternative parameter that incorporates information on the water surface slope (which can also be derived from ALB information), and thus indirectly on the energy dissipation, was suggested as a better indicator for roughness parameterizations in hydraulic models of Mountain River flow.