The paper presents the relationship between the erosion of the river bed and bankfull value. Based on the four years of observation of Porębianka stream, along the river trained part of the stream with cascade of block boulder ramps significant changes of river morphology and river bed were noticed. In. The aim of the present study was to determine local changes of river bed level and river water level and their influence on bankfull value. Bankfull was found using four classical methods and with implementation of HEC-RAS computer model. As a result we found that the erosion indicator in the cross section affected by the hydraulic structure is the increasing value of bankfull stage but only when the level of the bed is lower than the top sill level of the hydraulic structure. In other cases, the bankfull discharge is responsible.