Due to the very fine space resolution and high noise-reduction capability of the improved laboratory-made laser Doppler anemometer, the temporal mean velocity and velocity fluctuation in uniform, rough bed open channel turbulent flows were measured in full detail and a large amount of information about the wall region was obtained. The influence of boundary roughness on flow characteristics was examined by changing the relative smoothness scale, H/ks, of the flow. It is proposed that there are three sorts of bed roughness, namely, the large scale roughness, the mediuin one and the small one. The small scale roughness is of more universal significance to engineering practice, and much attention is devoted to the characteristics of the flow over boundaries of such a sort of roughness.