In the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations. Laboratory experiments are carried out to study a Y-shaped bifurcation with fixed banks and erodible bed composed of well-sorted sand. The problem is also analyzed by developing a theoretical, one-dimensional model. Results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node, which are strictly related to bar migration. The effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration speed. Four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers. The theoretical predictions are in good qualitative agreement with the experimental observations.