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Results of lattice Boltzmann simulations on the flow through narrow channels are presented. The focus of the work is the effect wall roughness on microscale flows at moderate Reynolds numbers, i.e. at Reynolds numbers significantly larger than unity (100<=Re<=2000), but not large enough in order to give rise to fully developed turbulence. It was shown in a previous work [1] that the presence of wall roughness may significantly alter the flow behavior, triggering a transition from a laminar to a time dependent flow. Further studies underlined the significance of the roughness wave length on the observed phenomenon [2]. In the present work, we study this issue further providing new support for the occurrence of flow instability as well as on the chaotic nature of the flow. In particular, via a study of probability density of the velocity field as well as spatial correlations between the velocity fluctuations along the flow direction, we demonstrate that wall roughness may give rise to a variety of situations such as simple oscillatory modes as well as fully chaotic flows. The latter point is further evidenced by visualizing the flow field via passive tracers.