In the study of natural rivers, accurate conveyance estimation is challenging because of complex cross-sectional geometry and variable flow resistance. The aim of this study was to investigate the additional friction factor due to lateral momentum transfer at the interface of vegetated and nonvegetated channel parts in two rivers: (1) the Rhine River, a large dredged river with partially vegetated floodplains, and (2) the River P¿nt¿neenjoki, a small boreal lowland river with dense bank vegetation and an undulating longitudinal profile. The friction factors of the interfaces were computed backward from the topographical field data, while the measured depth-flow regime data were computed with the help of an unsteady one-dimensional (1-D) flow model. Laboratory experiments were carried out to verify the effects of longitudinal spacing of plants on the friction factors. In the researched rivers the values of the friction factors at the interface were higher than the ones presented in the literature.