Mountain streams are characterized by coarse-bed materials, steep slopes, and low flow depth. In these type of streams, direct measurement of discharges and/or flow velocities is often difficult, particulaly in high flows. Therefore, the use of flow resistance relationships will be inevitable. The present research aims to develop dimensionless relationships of hydraulic geometry in estimating the discharge without measuring the water depth. In this study, flow resistance in three mountain streams, including Ansarood, SaeedAbad and Lighvan with various bed-forms was investigated based on measured data, and a new equation was proposed. The present study results revealed that in the case of mountain rivers, the inclusion of the geometric standard deviation of bed materials in the non-dimensional hydraulic geometry relationships increases the correlation coefficient and the accuracy of the predictive equations.