The effects of a transverse flow on the dynamics of a directional solidification interface are studied experimentally in a thin sample. The set-up enables a non-intrusive visualization of the interface and an independent control of both the flow and the solidification conditions. The flow is forced in the sample from an external thermosiphon which provides an accurate steady velocity up to 1.2 minis. A transparent melt of succinonitrile is used with a sample depth allowing the solidification of a single layer of microstructures. Downstream inclinations of microstructures and downstream promotion of dendritic sidebranching are observed. Surprisingly, large scale traveling waves involving a wavelength of several cells or dendrites are evidenced on the interface in a large range of conditions. Two kinds of waves are evidenced, one involving a slow velocity, a weak amplitude and a sinusoidal profile, the other a large velocity, a large amplitude and a non-linear profile. Both result from the coupling between solidification and flow and induce striations in the solid phase. (C) 2014 Elsevier B.V. All rights reserved