A dam stores surplus flood water and distributes this additional water through the power / irrigation tunnels in the dam or through the canals from its reservoir. In both cases it is the stored water that is distributed, and not river water which continues to flow normally as before.
It is at the barrages, for there is neither reservoir nor storage, that the canals take water directly from the rivers. Simply it can be differentiated that Dams add water , but barrages subtract it.
