An along estuary negative circulation was measured in the Ria of Pontevedra (NW Spain) on May 13, 1998. This two-layered circulation pattern, with near bed water moving seaward and near surface water landward, was generated by the existence of two different water masses at intermediate depths. Water in the inner part of the estuary was a mixture of estuarine water and Eastern North Atlantic Central Water (ENACW), which had entered the estuary some days before due to an intense upwelling event. Water in the middle part of the estuary was a mixture of estuarine water and fresh water coming from the river discharge, which had been higher during the days previous to May 13. This water mass distribution gave rise to the existence of denser water in the inner part of the estuary than in the middle one, generating the observed negative circulation pattern.