Abstract: A series of experiments were conducted in a rectangular flume by varying aspect ratio and sidewall roughness. The vertical velocity distributions over cross sections are analysed systematically. It is demonstrated that each velocity profile can be divided into a logarithmic velocity region in the lower part and a parabolic vciocity region in the upper part, according to its characteristics. The expressions of velocity distribution and the dividing line between the tworegions, as well as the variations of the location of maximum velocity, with respect to aspect ratio and sidewall roughness, are given herein. Moreover, the variation of resistance coefficients for smooth and rough open-channel flows, the separation of bed and sidewall resistance coefficients, and the distributions of bed and sidewall shear stresses, are also discussed.