A study on the friction in unsteady open-channel flow over gravel beds is presented in this paper. Different hydrographs (Table 1) were experimentally investigated. The friction velocity was determined using the equation of mot ion (equation 1). The unsteadiness of the flow was expressed with thedimensionlessClauser parameter (equation 17). It is shown that for a given hydrograph the friction velocity,u*SVis usually larger in the rising branch (accelerating flow) than in the falling branch (decelerating flow), as can be seen in Fig. 3; it reaches its maximum value before the arrival of the peak (D=Dmax) of the hydrograph. During the passage of a hydrograph, the friction coefficient,fis usually larger in the rising branch and smaller in the falling branch; the descending tendency of thef-value in the rising branch is usually followed by a less pronounced ascending one in the falling branch (Fig. 7). The findings on the friction coefficient,f, could be used to understand the field data reported in the literature (Fig. 8).