Between 1971 and 1986, a number of major works from the Delta Plan were finished in the delta region: in 1971, the sluices in the mouth of the Harlingvliet and the Brouwersdam, closing off the Grevelingen from the sea and in 1986, the storm surge barrier in the mouth of the Oosterschelde. The construction of these works led to a number of significant changes in the Voordelta. Through the disappearance of the incoming and outgoing tidal flows, wave action has reverted the former shoals into exposed banks. A sheltered shallow lagoon-like region has formed behind these banks, rich in benthic life. Enhanced growth and formation of tidal flats and young dunes occur along the coast. The importance of the Voordelta for seals, birds and juvenile fish is rapidly increasing thanks to these changes, which are unique in the Netherlands with respect to size and speed.
In the mid 1990s, the development in the Voordelta reached an equilibrium. Since then, changes seem to be related to 'normal' dynamics of the coastal zone, such as temporary changes in tidal currents and/or storms.
In 1997, 63 harbour seals were counted in the Voordelta, including several young. In 2007, a total of 148 grey and harbour seals were counted in the Voordelta and 24 (including a couple of young) in the Oosterschelde. The Voordelta is particularly important as bird areas for (foraging) gulls, terns, spoonbills and (overwintering) greater scaup.
