This study presents experimental results on the effects of streamwise-oriented riblets on the coherent structures of turbulence. The riblet spacings correspond to 0.5 and 1.5 times the natural spacing of the low-speed streaks. The cross-sectional dimensions of the riblets were chosen to promote more effective control on the development and spatial distribution of wave packets consisting of hairpin vortices. Of the two riblet spacings considered, the wider spacing demonstrated effective control on the spanwise positioning of the wave packets, enabling reduction in their mutual interaction and realizing a reduction in their spanwise density compared to the conditions on a smooth surface. The closer-spaced riblets were observed to have even more control on the spanwise positioning of the wave packets, and produced notably stronger sweep and ejection events while promoting mutual interaction of hairpin vortices in spanwise-adjacent wave packets via spanwise-oriented vortical structures created by a Kelvin-Helmholtz instability mechanism.