Powerful alluvial rivers in the northern Alborz mountain ranges erode river banks due to having high slopes. Most of these rivers flow in forest areas. In this research, the rate of the river bank erosion was examined using the exposed roots of the trees. For this purpose, 8 reaches of Lavij Stream were investigated. To determine the first year of root exposure, two sets of macroscopic and microscopic indicators were utilized. Accordingly, the rate of the stream bank erosion was estimated. The results were analyzed by using statistical test, which showed insignificant differences between the two groups of indicators. Due to its more abundance (frequency) on the margins and easy detection of its root through the exposure (macroscopic and microscopic) indicators, Alnus glutinosa (black alder) species could be more easily and accurately analyzed as compared with any other tree species in the region. The mean erosion rate of the riverbank using the extruded roots was estimated to be 0.08 m/yr. The hydrological analyses of flood flows showed that 95% of Lavij Stream bank erosion was caused by the river bank full discharges with a return period of 1-3 years.
