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This paper presents results of the friction factor of cobble-bed and boulder-bed rivers based on 23 field measurements in three mountainous rivers in Iran.Results reveal that features of our measured velocity profiles in boulder-bed rivers in mountainous region do not present a specific shape but illustrate specific pattern in cobble-bed rivers.This difference is attributed to scale effect of relative submergence.It is found that the relative submergence in cobble and boulders-bed rivers is the only key parameter that influences friction factor estimation and other parameters such as particle Froude number and Froude number have few impacts on flow resistance of cobble-bed and boulder-bed rivers.The proposed relationship for estimation for the friction factor should be useful for large coarse-bed rivers.