The present study aimed to describe the hydrological functioning of the Northern branch of the Red River estuary (Song Hong). This research results obtained from the project "Hydrodynamic and anthropogenic influences on phytoplankton and bacterioplankton of the Bach Dang estuary"(2008-2010) funded by VAST, IRD and the French National Program EC2CO. A very contrasted functioning was observed between wet and dry seasdns. During the wet season, the strong freshwater discharge dominated the flow, producing high turbulence levels. During the dry season, as the steady fluvial component weakened, the action of tidal intrusion became predominant with a significant action of tidal pumping during the dry season which is responsible of a strong silting up of the estuarine system.