This study investigates experimentally the effect of suspended load on the roughness coefficient in a gravel bed river located at the northern Iran. The experiments were carried out during March and April when the suspended load is active in the river. After selecting three suitable reaches and ten cross sections to collect data, the velocity profiles, particle size characteristics and suspended load were measured simultaneously. The log law along with the boundary-layer characteristics methods were applied to determine shear velocity and roughness coefficient. The results revealed that the log law may apply under suspended load condition, however, its elements such as shear velocity and von Karman constant are not fixed and change based on flow conditions. The von Karman Constant decreases with the increase of suspended load and reaches 0.3 which is considerable under the Keywords: Gravel-bed river, suspended load, von Karman constant, roughness coefficient, Babolroud
