Bank vegetation substantially influences flow resistance, velocity, shear stress distributions, and geomorphic stability in many natural river settings. We analyze field data from gravel bed streams with typed bank vegetation characteristics and employ three-dimensional computational fluid dynamics (CFD) modeling to examine whether the effects of bank vegetation on channel form and processes are scale dependent. Field data from the United States and United Kingdom indicate that mean bankfull

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... ensionless shear stresses are significantly higher in channels with thick woody vegetation but only for channel widths less than ~20 m. Because specific mechanisms controlling the apparent scale dependency are difficult to isolate in natural channels, we develop CFD models of streams with coarse beds and bank vegetation to investigate physical processes in channels with variable bed and bank roughness. The CFD models are applied in two sets of simulations to improve mechanistic understanding of patterns in the field data and to examine (1) spatial scale dependency between channel width and vegetation effects and (2) the coevolution of flow hydraulics, channel form, and vegetation establishment. The scale-dependent bank vegetation effects on shear stress distributions in the CFD representations are consistent with field data from gravel bed streams and suggest that the length scale of bank vegetation protrusion relative to channel width is an important factor that could improve shear stress partitioning models. In general, the field data and CFD simulations indicate a significant scale-dependent effect of bank vegetation with important implications for stream restoration designs based on tractive force, regime, and analytical approaches.