A series of detailed measurements were made of unidirectional turbulent open-channel flow over fixed, artificial, sinuous-crested three-dimensional (3-D) dune shapes. The response of the mean free surface was two-dimensional (2-D). Mean streamwise velocities were largest over the node of the sinuous dune crest line rather than over the maxima or minima of the crest line. Friction coefficients of the 3-D dunes were 50% higher on average in comparison to those of their 2-D counterparts when subjected to flows of similar depths and discharges. Despite the larger friction of the 3-D dunes, the turbulence generated by them was much weaker than that generated by their 2-D counterparts. The spatial structure of turbulence caused by the sinuous crest line of the 3-D dunes was correlated with the presence of secondary currents. These secondary current cells directed flow upward in the regions where the streamwise velocities were low and downward in the regions where streamwise velocities were high, suggesting that they could be responsible for a significant amount of momentum flux not being carried by the low turbulent stresses over the 3-D dunes.