Smooth-walled open channel flow datasets, covering both the direct numerical simulation and experimental measurements with a friction Reynolds number
${\textit {Re}}_\tau$ at a low-to-moderate level of
$550\sim 2400$, are adopted to investigate the contributions of different scale motions to the mean wall-shear stress in open channel flows (OCFs). The FIK identity decomposition method by Fukagata et al. (Phys. Fluids, vol. 14, 2002, L73) combined with a scale decomposition is chosen for this research. To see whether/how the contributions in OCFs differ with those in closed channel flows (CCFs), comparisons between the two flows are also made. The scale-decomposed ‘turbulent’ contribution results of present OCFs exhibit two dominant contribution modes (i.e. large-scale motions (LSMs) and very-large-scale motions (VLSMs)) at a streamwise wavelength
$\lambda _x=1\sim 2h$ and
$O(10h)$, where
$h$ is the water depth. The large scales with
$\lambda _x>3h$ and
$\lambda _x>10h$ are demonstrated to contribute to over 40 % and 20 % of the mean wall-shear stress, respectively. Compared with CCFs, slightly higher and lower contributions in the
$\lambda _x>O(10h)$ and
$\lambda _x < O(10h)$ wavelength ranges are observed in OCFs, revealing the important free-surface effects in OCFs. Possible mechanisms are discussed to lend support for the observed differences between the two flows.