Vegetation plays a fundamental role in riverine environments, by affecting both hydrodynamics and morphodynamics. At the same time, flow velocity and sediment scouring influence the decay of plants by uprooting. The balance among such interactions defines whether or not rivers are colonized by submerged or emergent vegetation. Previous studies focused on the shift between vegetated and barebed conditions through flume experiments or numerical simulations. Herein, we derive analytical formulations for the threshold in terms of flow velocity and Froude number, by accounting for the conditions of submergence. Both the formulations predict lower thresholds for submerged vegetation than emergent plants. Vegetation characteristics and flow regime variability play the major role in controlling the thresholds. The comparison of the proposed relationships to available data shows a good agreement. These results have important implications to understand bio-morphological changes induced by natural and human factors, as well as to design effective river restoration projects.