Open-channel flows are an essential part of a wide range of human activities as well as of natural ecosystems. In water streams with mobile beds (e.g. gravel or sand bed rivers), it may be possible to observe the appearance of streamwise ridges with spanwise spacings of approximately two flow depths, which are capable of generating time-averaged streamwise vortices denoted as secondary currents. The aim of this work is therefore to study the possible effects of streamwise ridges on hydraulic resistance and flow structure in open-channel flows. Two sets of laboratory experiments were performed. The first consisted of bulk hydraulic resistance measurements for a range of channel bed slopes, flow depths and ridge spacings. In the second set, long-duration velocity measurements were carried out for a range of ridge spacings keeping the other parameters the same. A four-camera particle image velocimetry system in stereoscopic configuration was used to measure all three velocity components continuously for two hours. The results revealed a relationship between the friction factor and the ridge spacing normalised by the flow depth. Analysis of the velocity measurements suggested that this finding is mainly due to the presence of ridge-induced secondary currents, with sizes proportional to the spanwise spacing of the ridges. Such secondary currents were also found to prevent the formation of depth-scale turbulent structures denoted as very-large-scale motions. For a subset of relative ridge spacings, a new spectral feature denoted herein as 'secondary current instability' appears, possibly representing flow instabilities associated with inflection points in the mean velocity transverse distributions. Finally, the analysis of double-averaged velocity statistics provided further insights on the effects that streamwise ridges have on the flow.