Waters flowing in natural streams and rivers have the ability to scour and to deposit materials, hence to change the bed topography. It is recognized that undular flows have great potential for sediment transport. In the present study, a fixed-bed model was used to investigate the spatial variations of boundary shear stress under standing waves (i.e., undular flow). The results (Figure 8) highlight the nonuniformity of the boundary shear stress distributions. Minimum boundary shear stress is observed under the wave crests, and maximum shear stress is observed under the wave troughs. The experimental findings suggest the formation of three-dimensional standing waves bed forms. Overall, the study highlights large variations of boundary shear stress in response to free-surface undulations. ¿ 2000 American Geophysical Union