For overland flows transporting predominantly bed load over rough mobile beds without rainfall, resistance to flow *f *may be divided into four components: surface resistance *fs*, formresistance *ff*, wave resistance *fw*, and bed-mobility resistance *fm*. In this study it is assumed that *f *= *fs *+ *ff *+ *fw *+ *fm*, and an equation is developed for each component. The equations for*fs *and *ff *are borrowed from the literature, while those for *fw *and *fm *are developed from two series of flume experiments in which the beds are covered with various concentrations oflarge-scale roughness elements. The first series consists of 65 experiments on fixed beds, while the second series contains 194 experiments on mobile beds. All experiments wereperformed on the same slope (*S *= 0·114) and with the same size of sediment (*D *= 0·00074 m). The equations for *fw *and *fm *are derived by a combination of dimensional analysis andregression analysis. The analyses reveal that the major controls of *fw *and *fm *are the Froude number *F *and the concentration of the roughness elements *Cr*. When the equations for *fw*and *fm *are summed, the *Cr *terms cancel out, leaving *fw*+*m *= 0·63*F*-2. An equation is developed that predicts total *f*, and the contributions of *fs*, *ff*, *fw *and *fm *to *f *are computed from the series1 and 2 experiments. An analysis of the first series reveals that in clear-water flows over fixed beds, *fw *accounts for 52 per cent of *f*. A similar analysis of the second series indicates that insediment-laden flows over mobile beds *fw *comprises 37 per cent and *fm *32 per cent of *f*, so that together *fw *and *fm *account for almost 70 per cent of *f*. Finally, regression analysesindicate that where *F *>·5, *fw *and *fm *each vary with *F *-2 and *fw/fm *= 1·18. The equation developed here for predicting total *f *applies only to the range of hydraulic, sediment, andbed roughness conditions represented by the experimental data. With additional data from a broader range of conditions the same methodology as employed here could be used to developa more general equation. Copyright © 2006 John Wiley & Sons, Ltd.